Add note to merge commit email selector about rebase and squash merges

Author: greysteil

content/github/collaborating-with-issues-and-pull-requests/merging-a-pull-request.md

@@ -45,9 +45,15 @@ If you decide you don't want the changes in a topic branch to be merged to the u
 
 {% data reusables.files.choose-commit-email %}
 
+    {% note %}
+
+    **Note:** The email selector is not available for rebase merges, which do not create a merge commit, or for squash merges, which credit the user who created the pull request as the author of the squashed commit.
+
+    {% endnote %}
+
 6. Click **Confirm merge**, **Confirm squash and merge**, or **Confirm rebase and merge**.
 6. Optionally, [delete the branch](/articles/deleting-unused-branches). This keeps the list of branches in your repository tidy.
-   
+
 The repository may be configured so that the head branch for a pull request is automatically deleted when you merge a pull request. For more information, see "[Managing the automatic deletion of branches](/github/administering-a-repository/managing-the-automatic-deletion-of-branches)."
 
    {% if currentVersion == "free-pro-team@latest" or currentVersion ver_gt "[email protected]" %}