Doc fix for git-reflog: mention @{...} syntax, and <ref> in synopsys.

Doc fix for git-reflog: mention @{...} syntax, and <ref> in synopsys.

The HEAD@{...} syntax was documented in git-rev-parse manpage, which
is hard to find by someone looking for the documentation of porcelain.
git-reflog is probably the place where one expects to find this.

While I'm there, "git revlog show whatever" was also undocumented.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>