Clarify some event types

This PR adds a bit more information about certain [event types](http://developer.github.com/v3/activity/events/types/):
* lists possible values for the "action" attribute (GollumEvent and IssueCommentEvent)
* explains that some events are no longer created, even though they still might be in users' timelines (GistEvent, DownloadEvent, ForkApplyEvent)
* adds a note that the TeamAddEvent is created in organization timelines, not user timelines
* explains when events are created, with links to other parts of API documentation