Added docs for `React.isValidClass` and `React.isValidComponent`.
Still missing top level api:
* `constructAndRenderComponent`
* `constructAndRenderComponentById`
* `withContext`
This order should make more sense; it moves important functions like React.renderComponent up and deprecated/discouraged ones like transferPropsTo and setProps down. No content changes.
Recently learned that components passed into `React.renderComponent` may not be the ones actually mounted. Also learned that it returns the mounted component. Added some documentation describing this.