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

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>