Add docs for MailRoom supporting alternative Azure deployments

Previously MailRoom could only be configured with Microsoft Graph for
the default Azure deployment. However, there are currently three other
national clouds supported in Azure
(https://docs.microsoft.com/en-us/graph/deployments).

This commit documents support for the `graph_endpoint` and
`azure_ad_endpoint` to enable support for these alternative Azure
deployments.

doc/administration/incoming_email.md

@@ -766,6 +766,8 @@ This example for Omnibus GitLab assumes you're using the following mailbox: `inc
 
 ##### Configure Microsoft Graph
 
+> Alternative Azure deployments [introduced](https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/5978) in GitLab 14.9.
+
 ```ruby
 gitlab_rails['incoming_email_enabled'] = true
 
@@ -788,4 +790,19 @@ gitlab_rails['incoming_email_inbox_options'] = {
 }
 ```
 
+For Microsoft Cloud for US Government or [other Azure deployments](https://docs.microsoft.com/en-us/graph/deployments), configure the `azure_ad_endpoint` and `graph_endpoint` settings.
+
+- Example for Microsoft Cloud for US Government:
+
+```ruby
+gitlab_rails['incoming_email_inbox_options'] = {
+   'azure_ad_endpoint': 'https://login.microsoftonline.us',
+   'graph_endpoint': 'https://graph.microsoft.us',
+   'tenant_id': '<YOUR-TENANT-ID>',
+   'client_id': '<YOUR-CLIENT-ID>',
+   'client_secret': '<YOUR-CLIENT-SECRET>',
+   'poll_interval': 60  # Optional
+}
+```
+
 The Microsoft Graph API is not yet supported in source installations. See [this issue](https://gitlab.com/gitlab-org/gitlab/-/issues/326169) for more details.

doc/user/project/service_desk.md

@@ -241,7 +241,8 @@ The configuration options are the same as for configuring
 
 ##### Microsoft Graph
 
-> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/214900) in GitLab 13.11.
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/214900) in GitLab 13.11.
+> - Alternative Azure deployments [introduced](https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/5978) in GitLab 14.9.
 
 Service Desk can be configured to read Microsoft Exchange Online mailboxes with the Microsoft
 Graph API instead of IMAP. Follow the [documentation in the incoming email section for setting up an OAuth2 application for Microsoft Graph](../../administration/incoming_email.md#microsoft-graph).
@@ -263,6 +264,22 @@ Graph API instead of IMAP. Follow the [documentation in the incoming email secti
   }
   ```
 
+For Microsoft Cloud for US Government or [other Azure deployments](https://docs.microsoft.com/en-us/graph/deployments), configure the `azure_ad_endpoint` and `graph_endpoint` settings.
+
+- Example for Microsoft Cloud for US Government:
+
+```ruby
+  gitlab_rails['service_desk_email_inbox_options'] = {
+   'azure_ad_endpoint': 'https://login.microsoftonline.us',
+   'graph_endpoint': 'https://graph.microsoft.us',
+   'tenant_id': '<YOUR-TENANT-ID>',
+   'client_id': '<YOUR-CLIENT-ID>',
+   'client_secret': '<YOUR-CLIENT-SECRET>',
+   'poll_interval': 60  # Optional
+  }
+}
+```
+
 The Microsoft Graph API is not yet supported in source installations. See [this issue](https://gitlab.com/gitlab-org/gitlab/-/issues/326169) for more details.
 
 #### Configuring a custom email address suffix