rerere.txt: Document forget subcommand

dea4562b (rerere forget path: forget recorded resolution, 2009-12-25)
introduced the forget subcommand for rerere.

Document it.
Signed-off-by: NMichael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-rerere.txt

@@ -7,7 +7,7 @@ git-rerere - Reuse recorded resolution of conflicted merges
 
 SYNOPSIS
 --------
-'git rerere' ['clear'|'diff'|'status'|'gc']
+'git rerere' ['clear'|'forget' [<pathspec>]|'diff'|'status'|'gc']
 
 DESCRIPTION
 -----------
@@ -40,6 +40,11 @@ This resets the metadata used by rerere if a merge resolution is to be
 aborted.  Calling 'git am [--skip|--abort]' or 'git rebase [--skip|--abort]'
 will automatically invoke this command.
 
+'forget' <pathspec>::
+
+This resets the conflict resolutions which rerere has recorded for the current
+conflict in <pathspec>.  The <pathspec> is optional.
+
 'diff'::
 
 This displays diffs for the current state of the resolution.  It is