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

include/core/SkCanvas.h

@@ -672,6 +672,11 @@ public:
         bitmap's original width/height, then the bitmap will be drawn as if it
         were in a Shader with CLAMP mode. Thus the color outside of the original
         width/height will be the edge color replicated.
+
+        If a shader is present on the paint it will be ignored, except in the
+        case where the bitmap is kA8_Config. In that case, the color is
+        generated by the shader.
+
         @param bitmap   The bitmap to be drawn
         @param left     The position of the left side of the bitmap being drawn
         @param top      The position of the top side of the bitmap being drawn

include/core/SkShader.h

@@ -334,6 +334,9 @@ public:
      *  exceed implementation limits (currently at 64K - 1)) then SkEmptyShader
      *  may be returned.
      *
+     *  If the src is kA8_Config then that mask will be colorized using the color on
+     *  the paint.
+     *
      *  @param src  The bitmap to use inside the shader
      *  @param tmx  The tiling mode to use when sampling the bitmap in the x-direction.
      *  @param tmy  The tiling mode to use when sampling the bitmap in the y-direction.