I added the following text to the 'overview' topic:
Note that controller classes, and the actions they contain, are URL addressable. Hitting the following URL creates an instance of the controller class foo.MyControllerClass and runs its begin action. (When no other action is specified, the begin method is run by default.)
Hitting the following URL creates an instance of foo.MyControllerClass (if it doesn't already exist) and invokes the someAction action. Note that the controller class isn't mentioned by name: it's assumed that only one controller class exists in the directory, so there is only one candidate controller class to instantiate.