Describe actor in Flipper.register
Describe actor in Flipper.register

The documentation doesn't make it clear that the `actor`'s class when checking a group isn't the same as the the class of the actor used in the arguments. 

This change is a small documentation to help clear up that misconception.
jnunemaker/flipper,jnunemaker/flipper,jnunemaker/flipper,jnunemaker/flipper