docs: Resolve issues with 'events'

docs: Resolve issues with 'events'

Four things to change here:

- The response is any array that can contain any type of event, not one
  of them.
- The 'actor' field is nullable.
- The 'cover' field of the 'cover-created' event is an embedded cover
  letter, not a string.
- The specifications for the 'current_delegate' and 'previous_delegate'
  fields of the 'patch-delegated' field were apparently invalid.

Signed-off-by: Stephen Finucane <stephen@that.guru>