trailer: finish formatting unification

trailer: finish formatting unification

Rename format_trailer_info() to format_trailers(). Finally, both
interpret-trailers and format_trailers_from_commit() can call
"format_trailers()"!

Update the comment in <trailer.h> to remove the (now obsolete) caveats
about format_trailers_from_commit(). Those caveats come from
a388b10fc1 (pretty: move trailer formatting to trailer.c, 2017-08-15)
where it says:

    pretty: move trailer formatting to trailer.c

    The next commit will add many features to the %(trailer)
    placeholder in pretty.c. We'll need to access some internal
    functions of trailer.c for that, so our options are either:

      1. expose those functions publicly

    or

      2. make an entry point into trailer.c to do the formatting

    Doing (2) ends up exposing less surface area, though do note
    that caveats in the docstring of the new function.

which suggests format_trailers_from_commit() started out from pretty.c
and did not have access to all of the trailer implementation internals,
and was never intended to replace (unify) the formatting machinery in
trailer.c. The refactors leading up to this commit (as well as
additional refactors that will follow) expose additional functions
publicly, and is therefore choosing option (1) as described in
a388b10fc1.

Signed-off-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>