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

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

@@ -45,6 +45,12 @@ 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.