trailer: document parse_trailers() usage

trailer: document parse_trailers() usage

Explain how to use parse_trailers(), because earlier we made the
trailer_info struct opaque. That is, because clients can no longer peek
inside it, we should give them guidance about how the (pointer to the)
opaque struct can still be useful to them.

Rename "head" struct to "trailer_objects" to make the wording of the new
comments a bit easier to read (because "head" itself doesn't really have
any domain-specific meaning here).

Signed-off-by: Linus Arver <linus@ucla.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>