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
$ hg init outer
$ cd outer
hg debugsub with no remapping
$ echo 'sub = http://example.net/libfoo' > .hgsub
$ hg add .hgsub
$ hg debugsub
path sub
source http://example.net/libfoo
revision
hg debugsub with remapping
$ echo '[subpaths]' > .hg/hgrc
$ printf 'http://example.net/lib(.*) = C:\\libs\\\\1-lib\\\n' >> .hg/hgrc
$ hg debugsub
path sub
source C:\libs\foo-lib\
revision
test cumulative remapping, the $HGRCPATH file is loaded first
$ echo '[subpaths]' >> $HGRCPATH
$ echo 'libfoo = libbar' >> $HGRCPATH
$ hg debugsub
path sub
source C:\libs\bar-lib\
revision
test bad subpaths pattern
$ cat > .hg/hgrc <<EOF
> [subpaths]
> .* = \1
> EOF
$ hg debugsub
abort: bad subrepository pattern in $TESTTMP/outer/.hg/hgrc:2: invalid group reference
[255]