Actualize the "@return" types for the "Api" class methods

[aik099]

Currently most of Api class methods have either Result|false or array|false return type specified.

I doubt that false is ever returned (e.g. for HTTP 2xx responses), and it's more likely that an exception would be thrown by the ClientInterface implementing class instead.

Need to check that fact for each Api class method and update their @return annotation accordingly.