Add documentation about 0 scale matrix to Shader

fix:36989035

Test: build
Change-Id: Id1134d5d31ea6a17dc56f24d26eb5da9a78a2c2d

graphics/java/android/graphics/Shader.java

@@ -80,7 +80,8 @@ public class Shader {
 
     /**
      * Set the shader's local matrix. Passing null will reset the shader's
-     * matrix to identity.
+     * matrix to identity. If the matrix has scale value as 0, the drawing
+     * result is undefined.
      *
      * @param localM The shader's new local matrix, or null to specify identity
      */