git-remote-helpers.txt: clarify options & ref list attributes

git-remote-helpers.txt: clarify options & ref list attributes

The documentation was misleading in that it gave the impression that
'for-push' could be used as a ref attribute in the output of the
'list' command. That is wrong.

Also, explicitly point out the connection between the commands
'list' and 'options' on the one hand, and the sections
'REF LIST ATTRIBUTES' and 'OPTIONS' on the other hand.

Signed-off-by: Max Horn <max@quendi.de>
Acked-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>