Document git reflog --stale-fix

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 1389d9ddaa68a4cbf5018d88f971b9bbb7aaa3c9
  "reflog expire --fix-stale"
which introduced this option.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>