avcodec: Fix Doxygen trailing brief comments

The //< comment is not any magic comment supported by Doxygen,
instead use ///< to mark them as doc for the members.

libavcodec/avcodec.h

@@ -2791,10 +2791,10 @@ int avcodec_get_hw_frames_parameters(AVCodecContext *avctx,
  */
 
 enum AVPictureStructure {
-    AV_PICTURE_STRUCTURE_UNKNOWN,      //< unknown
-    AV_PICTURE_STRUCTURE_TOP_FIELD,    //< coded as top field
-    AV_PICTURE_STRUCTURE_BOTTOM_FIELD, //< coded as bottom field
-    AV_PICTURE_STRUCTURE_FRAME,        //< coded as frame
+    AV_PICTURE_STRUCTURE_UNKNOWN,      ///< unknown
+    AV_PICTURE_STRUCTURE_TOP_FIELD,    ///< coded as top field
+    AV_PICTURE_STRUCTURE_BOTTOM_FIELD, ///< coded as bottom field
+    AV_PICTURE_STRUCTURE_FRAME,        ///< coded as frame
 };
 
 typedef struct AVCodecParserContext {

libavcodec/codec_par.h

@@ -37,10 +37,10 @@
 enum AVFieldOrder {
     AV_FIELD_UNKNOWN,
     AV_FIELD_PROGRESSIVE,
-    AV_FIELD_TT,          //< Top coded_first, top displayed first
-    AV_FIELD_BB,          //< Bottom coded first, bottom displayed first
-    AV_FIELD_TB,          //< Top coded first, bottom displayed first
-    AV_FIELD_BT,          //< Bottom coded first, top displayed first
+    AV_FIELD_TT,          ///< Top coded_first, top displayed first
+    AV_FIELD_BB,          ///< Bottom coded first, bottom displayed first
+    AV_FIELD_TB,          ///< Top coded first, bottom displayed first
+    AV_FIELD_BT,          ///< Bottom coded first, top displayed first
 };
 
 /**