Documentation/git-push: clarify the description of defaults

Documentation/git-push: clarify the description of defaults

We describe what gets pushed by default when the command line does
not give any <refspec> under the bullet point of <refspec>.

It is a bit unfriendly to expect users to read on <refspec> when
they are not giving any in the first place.  "What gets pushed" is
determined by taking many factors (<refspec> argument being only one
of them) into account, and is a property of the entire command, not
an individual argument.  Also we do not describe "Where the push
goes" when the command line does not say.

Give the description on "what gets pushed to where" upfront before
explaining individual arguments and options.

Also update the description of <refspec> to say what it is, what it
is used for, before explaining what shape it takes.

Signed-off-by: Junio C Hamano <gitster@pobox.com>