add documentation for `on_bot_message`

[topher200]

The purpose of on_bot_message or how to use them is never explained in the documentation. In fact, that function signature never appears in the limbo codebase! (just in plugins). It makes it really hard to figure out what that function is for.

We might also want to add some documentation in limbo.py:run_hook around what the datastructure of hooks looks like and how we dynamically decide which hook to call.

[llimllib]

Definitely, I just wrote that quickly to fix an issue and should have documented it. Thanks!