Add hints to revert documentation about other ways to undo changes

Add hints to revert documentation about other ways to undo changes

Based on its name, people may read the 'git revert' documentation when
they want to undo local changes, especially people who have used other
SCM's.  'git revert' may not be what they had in mind, but git
provides several other ways to undo changes to files.  We can help
them by pointing them towards the git commands that do what they might
want to do.

Cc: Daniel Barkalow <barkalow@iabervon.org>
Cc: Lea Wiemann <lewiemann@gmail.com>
Signed-off-by: Tarmigan Casebolt <tarmigan+git@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>