Calling method/hook - what data available?

I’m building my first extension to help me achieve something where only a hook will do, and am a little confused about what data is available by default and what needs to be fetched:

Hook is:

http://ellislab.com/expressionengine/user-guide/development/extension_hooks/module/comment/index.html#insert-comment-end

Data available is:

$data $comment_moderate $comment_id

Are there any docs telling me what should be available in $data (in the case of each hook)? I can see what’s in comment by printing it, but are there any explicit docs?