Garden in prerequisites for squashing

A chicken-and-egg problem. We needed to tell the user to be sure
squashing was enabled on the project, before they could configure
it at the merge request level.

doc/user/project/merge_requests/squash_and_merge.md

@@ -33,7 +33,14 @@ squash commits and merge commits.
 ## Set default squash options for a merge request
 
 Users with permission to create or edit a merge request can set the default squash options
-for a merge request. To do this:
+for a merge request.
+
+Prerequisites:
+
+- Your project must be [configured](#configure-squash-options-for-a-project) to allow or
+  encourage squashing.
+
+To do this:
 
 1. Go to the merge request and select **Edit**.
 1. Select or clear the **Squash commits when merge request is accepted** checkbox.
@@ -58,6 +65,10 @@ squash the commits as part of the merge process:
 > - [Enabled on GitLab.com and self-managed by default](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/39382) in GitLab 13.3.
 > - [Generally available](https://gitlab.com/gitlab-org/gitlab/-/issues/232536) in GitLab 13.8. Feature flag `squash_options` removed.
 
+Prerequisites:
+
+- You must have at least the Maintainer role for this project.
+
 To configure the default squashing behavior for all merge requests in your project:
 
 1. On the top bar, select **Main menu > Projects** and find your project.