fetch doc: cross-link two new negotiation options

Users interested in the fetch.negotiationAlgorithm variable added in
42cc7485a2 ("negotiator/skipping: skip commits during fetch",
2018-07-16) are probably interested in the related --negotiation-tip
option added in 3390e42adb ("fetch-pack: support negotiation tip
whitelist", 2018-07-02).

Change the documentation for those two to reference one another to
point readers in the right direction.

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

Documentation/config.txt

@@ -1500,6 +1500,8 @@ fetch.negotiationAlgorithm::
 	that never skips commits (unless the server has acknowledged it or one
 	of its descendants).
 	Unknown values will cause 'git fetch' to error out.
++
+See also the `--negotiation-tip` option for linkgit:git-fetch[1].
 
 format.attach::
 	Enable multipart/mixed attachments as the default for

Documentation/fetch-options.txt

@@ -57,6 +57,9 @@ commits reachable from any of the given commits.
 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].
 
 ifndef::git-pull[]
 --dry-run::