fetch: document the --negotiate-only option

fetch: document the --negotiate-only option

There was no documentation for the --negotiate-only option added in
9c1e657a8fd (fetch: teach independent negotiation (no packfile),
2021-05-04), only documentation for the related push.negotiation
option added in the following commit in 477673d6f39 (send-pack:
support push negotiation, 2021-05-04).

Let's document it, and update the cross-linking I'd added between
--negotiation-tip=* and 'fetch.negotiationAlgorithm' in
526608284a7 (fetch doc: cross-link two new negotiation options,
2018-08-01).

I think it would be better to say "in common with the remote" here
than "...the server", but the documentation for --negotiation-tip=*
above this talks about "the server", so let's continue doing that in
this related option. See 3390e42adb3 (fetch-pack: support negotiation
tip whitelist, 2018-07-02) for that documentation.

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