Added some docs in Grid to clarify when/why scroll offsets are updated
Added some docs in Grid to clarify when/why scroll offsets are updated

nicholasrq/react-virtualized,edulan/react-virtualized,edulan/react-virtualized,nicholasrq/react-virtualized,bvaughn/react-virtualized,bvaughn/react-virtualized,cesarandreu/react-virtualized,cesarandreu/react-virtualized