CallbackTimer.timer is undocumented

https://docs.enthought.com/pyface/timer.html introduces the `CallbackTimer`, but doesn't document how to use it, and the API documentation for `CallbackTimer` at https://docs.enthought.com/pyface/api/pyface.timer.timer.html#pyface.timer.timer.CallbackTimer doesn't show the methods available or describe their behaviour.

In particular, it's not clear:

- what the possible parameters of the `timer` method are, and what they do, and what's returned (presumably it's an instance of `CallbackTimer`, but it would be good to make that explicit)
- how to use `CallbackTimer` as a singleshot timer
- whether instantiating the timer also starts it, or whether it needs to be started manually
- how to cancel the timer, and what the behaviour is in that case (what happens when you call cancel on an already completed timer? is it possible for the cancel to succeed and the callback to be called anyway?)

Tangentially related: this is an example of a situation where it's really useful to have source links in the documentation - in many cases the source answers some unanswered questions. xref: enthought/traitsui#1398