hgext: more patchbomb documentation
+ Add a description of how to enable this extension
+ Note which command it provides (it's not very easy to guess
that "hgext.patchbomb" enables the "email" command, unless one
can read Python sources)
+ Expand the descriptions of the -n and -m options of "hg email".
+ Mention that formail is (commonly) part of the procmail package.