Small cleanup in CE

Better comments for:
* ve.ce.Document.getRelativeOffset,
* ve.ce.Document.getSiblingWordBoundary.
Convert ve.ce.Surface.getSelectionRect to a static method.
Moved getNodeAndOffset from ve.ce.Surface to ve.ce.Document.

Change-Id: Ic00221fa463205d04c9b52150c0dd15904493b1e
Merge "Small cleanup in CE"