Docs: clarify how to enable Squash and Merge

doc/user/project/merge_requests/squash_and_merge.md

@@ -65,14 +65,27 @@ meaningful commit messages and:
 ## Enabling squash for a merge request
 
 Anyone who can create or edit a merge request can choose for it to be squashed
-on the merge request form:
+on the merge request form. Users can select or unselect the checkbox at the moment
+they are creating the merge request:
 
 ![Squash commits checkbox on edit form](img/squash_edit_form.png)
 
-This can then be overridden at the time of accepting the merge request:
+After the merge request is submitted, Squash and Merge can still be enabled or disabled
+by editing the merge request description:
+
+1. Scroll to the top of the merge request page and click **Edit**.
+1. Scroll down to the end of the merge request form and select the checkbox
+**Squash commits when merge request is accepted**.
+
+This setting can then be overridden at the time of accepting the merge request.
+At the end of the merge request widget, next to the **Merge** button, the **Squash commits** checkbox
+can be either selected or unselected:
 
 ![Squash commits checkbox on accept merge request form](img/squash_mr_widget.png)
 
+Note that Squash and Merge might not be available depending on the project's configuration
+for [Squash Commit Options](#squash-commits-options).
+
 ## Commit metadata for squashed commits
 
 The squashed commit has the following metadata: