Add documentation for the message handlers
Add documentation for the message handlers

This documents the fedmsg consumer class and the handler methods it
implements. There are still several undocumented public functions and
methods, but this covers all the incoming message types and the general
actions taken for each message.

Signed-off-by: Jeremy Cline <b3f594e10a9edcf5413cf1190121d45078c62290@jcline.org>

fedora-infra/the-new-hotness,fedora-infra/the-new-hotness