Minor fixup to documentation of hooks in git-receive-pack.

Minor fixup to documentation of hooks in git-receive-pack.

Small additional changes to the cbb84e5d174cf33fd4dcf3136de50a886ff9a2e2
commit, which introduced documentation to pre-receive and post-receive:
 - Mention that stdout and stderr are equivalent.
 - Add one cross-section link and fix one other.
 - Fix information on advantages of post-receive over post-update.

Signed-off-by: Jan Hudec <bulb@ucw.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>