reflog: improve and update documentation

reflog: improve and update documentation

Revamp the "git reflog" usage documentation in the manpage and the
command help to match the current reality and improve its clarity:

* Add documentation for some options that had been left out.

* Group the subcommands and options more logically and move more
  common subcommands/options higher.

* Improve some explanations.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>