Update documentation around drawBitmap and shaders

include/core/SkCanvas.h

Index: include/core/SkCanvas.h
diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index 7bcc09a7290aa6327fc50a0edc5ba3d7f7cd1dd8..6c41680e3ed63ae01839e6a79cf37c5de9da5a3d 100644
--- a/include/core/SkCanvas.h
+++ b/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