Add some comments.

Also, minor tweak to SurfaceTextureLayer.

Change-Id: If616d5ee4e8226dd0e16c5dbb0e0f80db553110e
diff --git a/include/gui/ISurfaceComposer.h b/include/gui/ISurfaceComposer.h
index 50b2ad1..4862949 100644
--- a/include/gui/ISurfaceComposer.h
+++ b/include/gui/ISurfaceComposer.h
@@ -39,6 +39,10 @@
 class IDisplayEventConnection;
 class IMemoryHeap;
 
+/*
+ * This class defines the Binder IPC interface for accessing various
+ * SurfaceFlinger features.
+ */
 class ISurfaceComposer: public IInterface {
 public:
     DECLARE_META_INTERFACE(SurfaceComposer);