Update documentation around drawBitmap and shaders

R=reed@google.com

Author: djsollen@google.com

Review URL: https://codereview.chromium.org/105823004

git-svn-id: http://skia.googlecode.com/svn/trunk@12507 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed