git-rebase.txt: highlight backend differences with commit rewording

git-rebase.txt: highlight backend differences with commit rewording

As noted by Junio:
    Back when "git am" was written, it was not considered a bug that the
    "git am --resolved" option did not offer the user a chance to update
    the log message to match the adjustment of the code the user made,
    but honestly, I'd have to say that it is a bug in "git am" in that
    over time it wasn't adjusted to the new world order where we
    encourage users to describe what they did when the automation
    hiccuped by opening an editor.  These days, even when automation
    worked well (e.g. a clean auto-merge with "git merge"), we open an
    editor.  The world has changed, and so should the expectations.

Junio also suggested providing a workaround such as allowing --no-edit
together with git rebase --continue, but that should probably be done in
a patch after the git-2.26.0 release.  For now, just document the known
difference in the Behavioral Differences section.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>