Updated RenderEffect documentation

-Added documentation on RenderEffect usage
-Updated documentation regarding coordinate space
of dst rect to be within bounds of target
RenderNode
-Updated createBlendModeEffect documentation to
remove references to nullability since the parameters
are required to be non-null

Bug: 173657415
Test: N/A
Change-Id: Ief136650e40d1b820de9e175ac906f87c67ea864