doc: clarify post-receive hook behavior

doc: clarify post-receive hook behavior

The `githooks` documentation mentions that the post-receive hook
executes once after git-receive-pack(1) updates all references and that
it also receives the same information as the pre-receive hook on
standard input. This is misleading though because the hook only
executes once if at least one of the attempted reference updates is
successful. Also, while each line provided on standard input is in the
same format as the pre-receive hook, the information received only
includes the set of references that were successfully updated.

Update the documentation to clarify these points and also provide a
reference to the post-receive hook section of the `git-receive-pack`
documentation which has additional information.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>