base

Update view's alpha and layer docs

bug:8501661

Makes the performance issues and interaction with layer type/paint
more clear.

Additionally, corrects change from 47ab7d6612e2b5b8b66fb261dafef7c91264e173
to still allow displayList alpha to override layer paint

Change-Id: Ic94d75865700820489370461cd8ac9f9077a8d90