Document git reflog --stale-fix

Document --stale-fix, used in "git reflog expire --stale-fix --all"
to remove invalid reflog entries, to fix situation after running
non reflog-aware git-prune from an older git in the presence of
reflogs (see RelNotes-1.5.0.txt).

Based on description of commit 1389d9dd
  "reflog expire --fix-stale"
which introduced this option.
Signed-off-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-reflog.txt

@@ -39,6 +39,19 @@ the current branch. It is basically an alias for 'git log -g --abbrev-commit
 OPTIONS
 -------
 
+--stale-fix::
+	This revamps the logic -- the definition of "broken commit"
+	becomes: a commit that is not reachable from any of the refs and
+	there is a missing object among the commit, tree, or blob
+	objects reachable from it that is not reachable from any of the
+	refs.
++
+This computation involves traversing all the reachable objects, i.e. it
+has the same cost as 'git prune'.  Fortunately, once this is run, we
+should not have to ever worry about missing objects, because the current
+prune and pack-objects know about reflogs and protect objects referred by
+them.
+
 --expire=<time>::
 	Entries older than this time are pruned.  Without the
 	option it is taken from configuration `gc.reflogExpire`,