Animation documentation for ReactTransitionGroup

Update to the animation documentation for ReactTransitionGroup to clear the air on where one can use it.

If someone tries to use it off of React.addons.ReactTransitionGroup, which is undefined, instead of its real location, React.addons.TransitionGroup, they get a vague error about being unable to set defaultProps of undefined in the React createElement body.
Merge pull request #2555 from mfunkie/patch-1

Animation documentation for ReactTransitionGroup