doxyfy frame type comments and add some more comments

Originally committed as revision 12337 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -751,12 +751,12 @@ typedef struct AVPanScan{
 #define FF_BUFFER_TYPE_COPY     8 ///< Just a (modified) copy of some other buffer, don't deallocate anything.
 
 
-#define FF_I_TYPE  1 // Intra
-#define FF_P_TYPE  2 // Predicted
-#define FF_B_TYPE  3 // Bi-dir predicted
-#define FF_S_TYPE  4 // S(GMC)-VOP MPEG4
-#define FF_SI_TYPE 5
-#define FF_SP_TYPE 6
+#define FF_I_TYPE  1 ///< Intra
+#define FF_P_TYPE  2 ///< Predicted
+#define FF_B_TYPE  3 ///< Bi-dir predicted
+#define FF_S_TYPE  4 ///< S(GMC)-VOP MPEG4
+#define FF_SI_TYPE 5 ///< Switching Intra
+#define FF_SP_TYPE 6 ///< Switching Predicted
 #define FF_BI_TYPE 7
 
 #define FF_BUFFER_HINTS_VALID    0x01 // Buffer hints value is meaningful (if 0 ignore).