notify: rewrite user documentation
The main intent is to turn the reference help into a configuration walkthrough.
It also fix several things:
- Do not suggest to use it for commit notifications, it cannot work
- Fix notify.strip default value
- Mention that subscriptions can be setup in Mercurial configuration files
- Improve notify.strip and notify.domain documentation
$ 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