docs: clean up `--empty` formatting in git-rebase(1) and git-am(1)

docs: clean up `--empty` formatting in git-rebase(1) and git-am(1)

Both of these pages document very similar `--empty` options, but with
different styles. The exact behavior of these `--empty` options differs
somewhat, but consistent styling in the docs is still beneficial. This
commit aims to make them more consistent.

Break the possible values for `--empty` into separate sections for
readability. Alphabetical order is chosen for consistency.

In a future commit, we'll be documenting a new `--empty` option for
git-cherry-pick(1), making the consistency even more relevant.

Signed-off-by: Brian Lyles <brianmlyles@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>