Update top level API docs for completeness.

[zpao]

Running through https://github.com/facebook/react/blob/48281a17e4f460020fa7b9c6dc8169a1b867327c/src/core/React.js

• DOM
• PropTypes
• initializeTouchEvents
• createClass
• constructAndRenderComponent
• constructAndRenderComponentByID
• renderComponent
• renderComponentToString
• unmountComponentAtNode
• unmountAndReleaseReactRootNode (deprecated, mentioned in unmountComponentAtNode)
• isValidClass
• isValidComponent
• version