docs: Replace "first make sure..." comment with equivalent --only flag

This part of the git-comparison table was implying you need to manually
check and clear staged changes in order to only update the previous
commit's message. That isn't the case, though. As mentioned in the
`git-commit` documentation [1], this can be directly achieved by using
the `--only` flag with `--amend`:

> If this option is specified together with --amend, then no paths need
> to be specified, which can be used to amend the last commit without
> committing changes that have already been staged.

[1]: https://git-scm.com/docs/git-commit#Documentation/git-commit.txt--o

docs/git-comparison.md

@@ -239,8 +239,7 @@ parent.
     <tr>
       <td>Edit description (commit message) of the previous change</td>
       <td><code>jj describe @-</code></td>
-      <td><code>git commit --amend</code> (first make sure that nothing is
-          staged)</td>
+      <td><code>git commit --amend --only</code></td>
     </tr>
     <tr>
       <td>Temporarily put away the current change</td>