Some updates to refspec documentation

I have skimmed the git-push manpage many times over a period of
several months, without fully understanding it. Today I sat down and
figured it out. It took some experiments and a little reading of the
source.

Here I offer some updates to the refspec documentation, so that others
can hopefully learn it faster than I did. As I expect some changes to
be more controversial than others, I have split it up in several small
parts. Feel free to squash.

There is a risk that I have changed some things to be wrong. I wanted
to make this update while I was still a frustrated newbie.



Anders