document the jj equivalent of git reset --soft

i did the following things:

1. make some changes to the working copy
2. run `jj commit`
3. before i added a description, i decided i didn't want to commit yet. in git, the way to do this is to delete the contents of the editor; git sees that it is blank and aborts the commit. in jj, this doesn't work, because descriptions are allowed to be empty.

now i have the following jj state:
```
; jj log --stat
@  pokrsyvs github@jyn.dev 2024-02-06 21:48:17.000 -05:00 ddd6217f
│  (empty) (no description set)
│  0 files changed, 0 insertions(+), 0 deletions(-)
◉  lqqmzmku github@jyn.dev 2024-02-06 21:48:02.000 -05:00 HEAD@git b03bf3de
│  (no description set)
│  config/jj.toml | 2 +-
│  1 file changed, 1 insertion(+), 1 deletion(-)
◉  xqkmwvwp github@jyn.dev 2024-02-06 21:47:08.000 -05:00 master b673f97b
```
i want to undo the most recent commit, `lqqmzmku`. i'm used to doing this with `git reset --soft HEAD~`, which makes the parent `xqkmwvwp` and leaves the change to `config/jj.toml` on disk while removing it from the git history; basically `lqqmzmku` would go back to being the working copy.

i found that `jj move` does what i want, but it took a lot of trawling the options, it wasn't obvious, and i couldn't find `git reset` mentioned in the docs.

docs/git-comparison.md

@@ -211,6 +211,11 @@ parent.
       <td><code>git reset --hard</code> (same as abandoning a change since Git
           has no concept of a "change")</td>
     </tr>
+    <tr>
+      <td>Abandon the parent of the working copy, but keep its diff in the working copy</td>
+      <td><code>jj move --from @-</code></td>
+      <td><code>git reset --soft HEAD~</code></td>
+    </tr>
     <tr>
       <td>Discard working copy changes in some files</td>
       <td><code>jj restore &lt;paths&gt;...</code></td>