fetch: document the --negotiate-only option

There was no documentation for the --negotiate-only option added in
9c1e657 (fetch: teach independent negotiation (no packfile),
2021-05-04), only documentation for the related push.negotiation
option added in the following commit in 477673d (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
5266082 (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 3390e42 (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>

Documentation/config/fetch.txt

@@ -69,7 +69,8 @@ fetch.negotiationAlgorithm::
 	setting defaults to "skipping".
 	Unknown values will cause 'git fetch' to error out.
 +
-See also the `--negotiation-tip` option for linkgit:git-fetch[1].
+See also the `--negotiate-only` and `--negotiation-tip` options to
+linkgit:git-fetch[1].
 
 fetch.showForcedUpdates::
 	Set to false to enable `--no-show-forced-updates` in

Documentation/fetch-options.txt

@@ -62,8 +62,17 @@ The argument to this option may be a glob on ref names, a ref, or the (possibly
 abbreviated) SHA-1 of a commit. Specifying a glob is equivalent to specifying
 this option multiple times, one for each matching ref name.
 +
-See also the `fetch.negotiationAlgorithm` configuration variable
-documented in linkgit:git-config[1].
+See also the `fetch.negotiationAlgorithm` and `push.negotiate`
+configuration variables documented in linkgit:git-config[1], and the
+`--negotiate-only` option below.
+
+--negotiate-only::
+	Do not fetch anything from the server, and instead print the
+	ancestors of the provided `--negotiation-tip=*` arguments,
+	which we have in common with the server.
++
+Internally this is used to implement the `push.negotiate` option, see
+linkgit:git-config[1].
 
 --dry-run::
 	Show what would be done, without making any changes.