Move BVolumeRoster docs to Haiku Book.

With this commit every class in the storage kit is now documented
in the Haiku book!

Thanks to Ingo, Axel, Vincent Dominguez, Tyler Dauwalder, and
everyone who helped document these classes.

headers/os/storage/VolumeRoster.h

@@ -36,8 +36,16 @@ private:
 
 private:
 			int32				fCookie;
+									// The iteration cookie for next_dev()
+									// Initialized to 0
 			BMessenger*			fTarget;
+									// BMessenger referring to the target to
+									// which the watching notification
+									// messages are sent. The object is
+									// allocated and owned by the roster,
+									// or NULL if not watching.
 			uint32				_reserved[3];
+									// FBC
 };