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>

Documentation/git-revert.txt

@@ -15,6 +15,15 @@ Given one existing commit, revert the change the patch introduces, and record a
 new commit that records it.  This requires your working tree to be clean (no
 modifications from the HEAD commit).
 
+Note: 'git revert' is used to record a new commit to reverse the
+effect of an earlier commit (often a faulty one).  If you want to
+throw away all uncommitted changes in your working directory, you
+should see linkgit:git-reset[1], particularly the '--hard' option.  If
+you want to extract specific files as they were in another commit, you
+should see linkgit:git-checkout[1], specifically the 'git checkout
+<commit> -- <filename>' syntax.  Take care with these alternatives as
+both will discard uncommitted changes in your working directory.
+
 OPTIONS
 -------
 <commit>::