pretty.h: update and expand docstring for userformat_find_requirements()

pretty.h: update and expand docstring for userformat_find_requirements()

The comment only mentions "notes", but there are more fields now (and
I'm about to add another). Let's make it more general, and stick the
struct next to the function to make the list of possibilities obvious.

While we're touching this comment, let's also mention the behavior of
NULL, which some callers rely on (though in the long run, this global is
pretty nasty and probably should get moved into rev_info).

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