update comment on setShader to clarify alpha-bitmap behavior in bitmapshaders

BUG=skia:2293
R=bsalomon@google.com, yunchao.he@intel.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/203203005

git-svn-id: http://skia.googlecode.com/svn/trunk@13851 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed