upload-pack: document and rename --advertise-refs

upload-pack: document and rename --advertise-refs

The --advertise-refs documentation in git-upload-pack added in
9812f2136b3 (upload-pack.c: use parse-options API, 2016-05-31) hasn't
been entirely true ever since v2 support was implemented in
e52449b6722 (connect: request remote refs using v2, 2018-03-15). Under
v2 we don't advertise the refs at all, but rather dump the
capabilities header.

This option has always been an obscure internal implementation detail,
it wasn't even documented for git-receive-pack. Since it has exactly
one user let's rename it to --http-backend-info-refs, which is more
accurate and points the reader in the right direction. Let's also
cross-link this from the protocol v1 and v2 documentation.

I'm retaining a hidden --advertise-refs alias in case there's any
external users of this, and making both options hidden to the bash
completion (as with most other internal-only options).

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