pull doc: refer to a specific section in 'fetch' doc
The documentation for the `<refspec>` parameter in the `git fetch`
documentation refers to the section "CONFIGURED REMOTE-TRACKING
BRANCHES" in this same documentation page.
In the `git pull` documentation, let's also refer specifically to this
section instead of just linking to the `git fetch` documentation.
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>