doc/rev-list-options: explain "-g" output formats

doc/rev-list-options: explain "-g" output formats

We document that asking for HEAD@{now} will switch the
output to show HEAD@{timestamp}, but not that specifying
`--date` has a similar effect, or that it can be overridden
with HEAD@{0}. Let's do so.

These rules come from 794151e (reflog-walk: always make
HEAD@{0} show indexed selectors, 2012-05-04), though that is
simply the culmination of years of these heuristics growing
organically.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>