git-am.txt: clarify --abort behavior

Both Johannes and I assumed (perhaps due to familiarity with rebase)
that am --abort would return the user to a clean state.  However, since
am, unlike rebase, is intended to be used within a dirty working tree,
--abort will only clean the files involved in the am operation.

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

Documentation/git-am.txt

@@ -178,6 +178,8 @@ default.   You can use `--no-utf8` to override this.
 
 --abort::
 	Restore the original branch and abort the patching operation.
+	Revert contents of files involved in the am operation to their
+	pre-am state.
 
 --quit::
 	Abort the patching operation but keep HEAD and the index