doc: git-push: clarify intro

doc: git-push: clarify intro

From user feedback, 5 users are unsure what "ref" and/or "objects" means
in this context. 3 users said they don't know what "complete the refs"
means.

Many users also commented that receive hooks do not seem like the most
important thing to know about `git push`, and that this information
should not be the second sentence in the man page.

Use more familiar language to make it more accessible to users who do
not know what a "ref" is and move the "hooks" comment to the end.

Signed-off-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>