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