Add clarification to Service Desk setup/configuration docs.

Description

There configuration/setup required for the Service Desk to be visible in the project configuration that is not very clear in the documentation.

1. Set up incoming email for the GitLab instance. This must support email sub-addressing.

Proposal

> **Note:**
Service Desk is enabled on GitLab.com. If you're a
[Silver subscriber](https://about.gitlab.com/gitlab-com/),
you can skip the step 1 below; you only need to enable it per project.
 `incoming_email_address` must be configured with the `%{key}` placeholder and `incoming_email_enabled` needs to be enabled in your gitlab.rb
 gitlab_rails['incoming_email_address'] = 'gitlab-incoming+%{key}@example.com'
 gitlab_rails['i

Links / references

https://docs.gitlab.com/ee/user/project/service_desk.html

Internal ZD: https://gitlab.zendesk.com/agent/tickets/93072

~Documentation customer ~"service desk"