hooks doc: clarify when receive-pack invokes its hooks
The text meant to say that receive-pack runs these hooks, and only
because receive-pack is not a command the end users use every day
(ever), as an explanation also meantioned that it is run in response
to 'git push', which is an end-user facing command readers hopefully
know about.
This unfortunately gave an incorrect impression that 'git push'
always result in the hook to run. If the refs push wanted to update
all already had the desired value, these hooks are not run.
Explicitly mention "... and updates reference(s)" as a precondition
to avoid this confusion.
Helped-by: Christoph Michelbach <michelbach94@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>