Existing code was delegating to the instance with delegate
macro calls, or invoking the instance method to reach
the object and call its instance methods.
But the point is to have a clean class-level interface where
the thread local instance is hidden in the implementation.
References #11c6973.
References #10198.
* It focuses on how to use it.
* Removes some ambigueties in the original docs about whether the state is stored in the class.
* Documents it provides class-level accessors via method_missing.
* Documents that if the extended class has an initializer, it must accept no arguments.
Xavier Noria