Add missing documentation for extension hooks

This change adds docs for the following hooks:
* ActionHook (and its various subclasses)
* FileAttachmentThumbnailHook
* ReviewRequestDetailHook
* TemplateHook

I've arranged the hooks documentation alphabetically, since I'm not sure of a
better way. While I was doing this I noticed a little bit of weirdness in the
ActionHook __init__ (actions should have been an empty list, not an empty
dict).

Testing done:
Generated the HTML docs and checked that things made sense and cross-linked
correctly.

Reviewed at https://reviews.reviewboard.org/r/4744/