exthelper: improve docs to indicate what module vars are needed
I recently tried creating an extension "from scratch" using exthelper, and it
wasn't obvious that you needed these. I believe that a careful reading of one of
the comments would tell you that they were required, but it's easy to miss and
having the examples be "complete" is helpful.
Differential Revision: https://phab.mercurial-scm.org/D10295
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state