Add documentation on enabling admin mode

What does this MR do?

Adds documentation on enabling admin mode.

Related issues

! 331508

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.

doc/user/admin_area/settings/sign_in_restrictions.md

@@ -28,47 +28,79 @@ You can restrict the password authentication for web interface and Git over HTTP
 
 In the event of an external authentication provider outage, use the [GitLab Rails console](../../../administration/operations/rails_console.md) to [re-enable the standard web sign-in form](#re-enable-standard-web-sign-in-form-in-rails-console). This configuration can also be changed over the [Application settings REST API](../../../api/settings.md#change-application-settings) while authenticating with an administrator account's personal access token.
 
-## Admin Mode
+## Admin mode
 
 > [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/2158) in GitLab 13.10.
 
-When this feature is enabled, instance administrators are limited as regular users. During that period,
-they do not have access to all projects, groups, or the **Admin Area** menu.
+In admin mode, instance administrators are limited as regular users,
+and do not have access to all projects, groups, or the **Admin Area** menu.
 
-To access potentially dangerous resources, an administrator can activate Admin Mode by:
-
-- Selecting the *Enable Admin Mode* button
-- Trying to access any part of the UI that requires administrator access, specifically those which call `/admin` endpoints.
-
-The main use case allows administrators to perform their regular tasks as a regular
+In this mode, administrators can perform regular tasks as a regular
 user, based on their memberships, without having to set up a second account for
 security reasons.
 
-When Admin Mode status is disabled, administrative users cannot access resources unless
-they've been explicitly granted access. For example, when Admin Mode is disabled, they
-get a `404` error if they try to open a private group or project, unless
-they are members of that group or project.
+### Enable admin mode
+
+Administrators can enable admin mode though the API, the Rails console, or the UI.
+
+#### Use the API to enable admin mode
+
+Make the following request to your instance endpoint:
+
+```shell
+curl --request PUT --header "PRIVATE-TOKEN:$ADMIN_TOKEN" "<gitlab-url>/api/v4/application/settings?admin_mode=true"
+```
+
+Replace the `<gitlab-url>` with your instance URL.
+
+#### Use the Rails console to enable admin mode
+
+Open the [Rails console](../../../administration/operations/rails_console.md) and run the following:
+
+```ruby
+::Gitlab::CurrentSettings.update_attributes!(admin_mode: true)
+```
+
+#### Use the UI to enable admin mode
+
+To enable admin mode through the UI:
+
+1. On the top bar, select **Main menu > Admin**.
+1. On the left sidebar, select **Settings > General**.
+1. Expand **Sign-in restrictions**.
+1. In the **Admin Mode** section, select the **Require additional authentication for administrative tasks** checkbox.
+
+### Activate admin mode
+
+To activate admin mode and access potentially dangerous resources, an administrator:
+
+1. On the top bar, selects **Enable Admin Mode**.
+1. Tries to access any part of the UI that calls `/admin` endpoints, and so requires administrator access.
+
+When admin mode status is disabled or deactivated, administrators cannot access resources unless
+they've been explicitly granted access. For example, administrators get a `404` error
+if they try to open a private group or project, unless they are members of that group or project.
 
-2FA should be enabled for administrators and is supported for the Admin Mode flow, as are
-OmniAuth providers and LDAP auth. The Admin Mode status is stored in the active user
-session and remains active until it is explicitly disabled (it will be disabled
-automatically after a timeout otherwise).
+2FA should be enabled for administrators. 2FA, OmniAuth providers, and LDAP
+authentication are supported for the admin mode flow. Admin mode status is
+stored in the active user session and remains active until it is explicitly disabled,
+or is disabled automatically after a timeout.
 
-### Limitations of Admin Mode
+### Limitations of admin mode
 
-The following access methods are **not** protected by Admin Mode:
+The following access methods are **not** protected by admin mode:
 
 - Git client access (SSH using public keys or HTTPS using Personal Access Tokens).
 - API access using a Personal Access Token.
 
-In other words, administrators who are otherwise limited by Admin Mode can still use
+In other words, administrators who are otherwise limited by admin mode can still use
 Git clients, and access RESTful API endpoints as administrators, without additional
 authentication steps.
 
 We may address these limitations in the future. For more information see the following epic:
 [Admin Mode for GitLab Administrators](https://gitlab.com/groups/gitlab-org/-/epics/2158).
 
-### Troubleshooting Admin Mode
+### Troubleshooting admin mode
 
 If necessary, you can disable **Admin Mode** as an administrator by using one of these two methods: