doc: git-push: rewrite refspec specification

doc: git-push: rewrite refspec specification

From user feedback, there was a request for examples, as well as a
comment that one person found "If git push [<repository>] without
any <refspec> argument is set to update some ref at the destination
with <src> with remote.<repository>.push configuration variable..."
impossible to understand.

To make the section easier to navigate, create a list of every possible
refspec form, with examples for each form as well as 2 forms which were
previously missing (patterns and negative refspecs).

Made a few changes to use more familiar language, but ultimately
refspecs are a pretty advanced feature so I've mostly left the
terminology alone.

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