brz help email needs updates for Breezy

[Tim Powell]

Now that the (formerly known as) bzr-email hook is included with Breezy, it would be helpful to revise the help to be Breezy- (and brz-) specific. As it is now, it is a little confusing.

It should drop the section that describes using the plugin with bzr versions before 0.15!

I believe there are additional undocumented configuration options, such as post_commit_subject, which has $revision and $message variables, and post_commit_body.