bundle API: start writing API documentation

bundle API: start writing API documentation

There are no other API docs in bundle.h, but this is at least a
start. We'll add a parameter to this function in a subsequent commit,
but let's start by documenting it.

The "/**" comment (as opposed to "/*") signifies the start of API
documentation. See [1] and bdfdaa4978d (strbuf.h: integrate
api-strbuf.txt documentation, 2015-01-16) and 6afbbdda333 (strbuf.h:
unify documentation comments beginnings, 2015-01-16) for a discussion
of that convention.

1. https://lore.kernel.org/git/874kbeecfu.fsf@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>