Docs: document the difference between the admin user and the server admin

Type of issue

There are 2 types of GitLab admin users:

"Admin user": the user who has admin permissions for a given GL instance. Logs into the instance via UI and has access to the admin area.
"Server admin": the user who has access to the instance administration via server. Has access to gitlab.rb and other admin files not accessible from the UI.

Problem to solve

Document the difference between the users, clarify who has access to what.

Further details

Example: the customization settings are accessible for users with admin privilegdes, from the UI. But only the server admin can update and reconfigure GL.

Proposal

Document this either on https://docs.gitlab.com/ee/user/admin_area/ or https://docs.gitlab.com/ee/administration/ and crosslink both docs.

Who can address the issue

@gl-docsteam (either of us)