Add comments describing some more files.

content/base/src/nsObjectLoadingContent.cpp

@@ -35,6 +35,13 @@
  *
  * ***** END LICENSE BLOCK ***** */
 
+/*
+ * A base class which implements nsIObjectLoadingContent and can be
+ * subclassed by various content nodes that want to provide
+ * plugin/document/image loading functionality (eg <embed>, <object>,
+ * <applet>, etc).
+ */
+
 // Interface headers
 #include "imgILoader.h"
 #include "nsIContent.h"