git-fetch & config doc: link to the new PRUNING section

Amend the documentation for fetch.prune, fetch.<name>.prune and
--prune to link to the recently added PRUNING section.

I'd have liked to link directly to it with "<<PRUNING>>" from
fetch-options.txt, since it's included in git-fetch.txt (git-pull.txt
also includes it, but doesn't include that option). However making a
reference across files yields this error:

    [...]/Documentation/git-fetch.xml:226: element xref: validity
    error : IDREF attribute linkend references an unknown ID "PRUNING"

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

Documentation/config.txt

@@ -1398,7 +1398,8 @@ fetch.unpackLimit::
 
 fetch.prune::
 	If true, fetch will automatically behave as if the `--prune`
-	option was given on the command line.  See also `remote.<name>.prune`.
+	option was given on the command line.  See also `remote.<name>.prune`
+	and the PRUNING section of linkgit:git-fetch[1].
 
 fetch.output::
 	Control how ref update status is printed. Valid values are
@@ -2944,6 +2945,9 @@ remote.<name>.prune::
 	remove any remote-tracking references that no longer exist on the
 	remote (as if the `--prune` option was given on the command line).
 	Overrides `fetch.prune` settings, if any.
++
+See also `remote.<name>.prune` and the PRUNING section of
+linkgit:git-fetch[1].
 
 remotes.<group>::
 	The list of remotes which are fetched by "git remote update

Documentation/fetch-options.txt

@@ -74,6 +74,9 @@ ifndef::git-pull[]
 	line or in the remote configuration, for example if the remote
 	was cloned with the --mirror option), then they are also
 	subject to pruning.
++
+See the PRUNING section below for more details.
+
 endif::git-pull[]
 
 ifndef::git-pull[]