some events are no longer created
clarify event types actions
clarify when events are triggered
show the failure responses for release uploads
remove download creation instructions, the API is disabled.
ReleaseEvent is only triggered on published releases
Merge pull request #309 from github/clarify-event-types

Clarify some event types
Merge pull request #310 from github/clarify-release-upload-responses

Clarify release upload responses
Merge branch 'master' into document-status-hook
tweak the status event description