Adjust contributing-admins.rst (aio-libs#12292)

Author: Dreamsorcerer

docs/contributing-admins.rst

@@ -35,9 +35,10 @@ first merge into the newer release branch (e.g. 3.8 into 3.9) and then to master
 #. Switch to target branch: e.g. ``git checkout 3.9 && git pull``
 #. Start a merge: e.g. ``git merge 3.8 --no-commit --no-ff --gpg-sign``
 #. Carefully review the changes and revert anything that should not be included (most
-   things outside the changelog).
+   things outside the changelog). Use `git checkout 3.9 path/to/file` to revert files
+   without aborting the merge.
 #. To ensure change fragments are cleaned up properly, run: ``python tools/cleanup_changes.py``
-#. Commit the merge (must be a normal merge commit, not squashed).
+#. Complete the merge: `git merge --continue`.
 #. Push the branch directly to Github (because a PR would get squashed). When pushing,
    you may get a rejected message. Follow these steps to resolve: