Add documentation for the new rotated buffer subclasses. (bug 1409871 part 21, r=nical)

MozReview-Commit-ID: E2XejdUju9Q

--HG--
extra : rebase_source : cda331b55f4af893a0d9e81f405c527bedb07cb2

gfx/layers/RotatedBuffer.h

@@ -262,6 +262,13 @@ protected:
   bool          mDidSelfCopy;
 };
 
+/**
+ * RemoteRotatedBuffer is a rotated buffer that is backed by texture
+ * clients. Before you use this class you must successfully lock it with
+ * an appropriate open mode, and then also unlock it when you're finished.
+ * RemoteRotatedBuffer is used by ContentClientSingleBuffered and
+ * ContentClientDoubleBuffered for the OMTC code path.
+ */
 class RemoteRotatedBuffer : public RotatedBuffer
 {
 public:
@@ -301,6 +308,10 @@ private:
   RefPtr<gfx::DrawTarget> mTargetOnWhite;
 };
 
+/**
+ * DrawTargetRotatedBuffer is a rotated buffer that is backed by draw targets,
+ * and is used by ContentClientBasic for the on-mtc code path.
+ */
 class DrawTargetRotatedBuffer : public RotatedBuffer
 {
 public:
@@ -331,6 +342,10 @@ private:
   RefPtr<gfx::DrawTarget> mTargetOnWhite;
 };
 
+/**
+ * SourceRotatedBuffer is a rotated buffer that is backed by source surfaces,
+ * and may only be used to draw into other buffers or be read directly.
+ */
 class SourceRotatedBuffer : public RotatedBuffer
 {
 public: