Added documentation of stage/TU allocation

http://codereview.appspot.com/6199058/



git-svn-id: http://skia.googlecode.com/svn/trunk@3888 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/gpu/GrPathRenderer.h b/src/gpu/GrPathRenderer.h
index 6a6cea6..ff9fdbb 100644
--- a/src/gpu/GrPathRenderer.h
+++ b/src/gpu/GrPathRenderer.h
@@ -21,6 +21,10 @@
 
 /**
  *  Base class for drawing paths into a GrDrawTarget.
+ *
+ *  Derived classes can use stages GrPaint::kTotalStages through 
+ *  GrDrawState::kNumStages-1. The stages before GrPaint::kTotalStages
+ *  are reserved for setting up the draw (i.e., textures and filter masks).
  */
 class GR_API GrPathRenderer : public GrRefCnt {
 public: