docs: Clarify actor map and unmap methods

docs: Clarify actor map and unmap methods

ClutterActor stopped requiring to override the map and unmap virtual
functions some time ago.

Now that ClutterActor implements the Container interface, overriding map
and unmap to control the MAPPED state of the children is pretty much
going to be a source of bugs and misunderstandings.

Plus, the ordering of the unmap, destroy, dispose, and finalize calls
should be be documented properly.

The documentation should clarify all that.