refine the hls codec, add comments.

@@ -134,6 +134,9 @@ public:
     int nb_sample_units;
     SrsCodecSampleUnit sample_units[SRS_MAX_CODEC_SAMPLE];
 public:
+    /**
+    * whether the sample is video sample which demux from video packet.
+    */
     bool is_video;
     /**
     * CompositionTime, video_file_format_spec_v10_1.pdf, page 78.
@@ -191,19 +194,26 @@ private:
     SrsStream* stream;
 public:
     /**
-    * video specified
+    * metadata specified
     */
+    int             duration;
+    int             width;
+    int             height;
+    int             frame_rate;
     // @see: SrsCodecVideo
     int             video_codec_id;
+    int             video_data_rate; // in bps
+    // @see: SrsCod ecAudioType
+    int             audio_codec_id;
+    int             audio_data_rate; // in bps
+public:
+    /**
+    * video specified
+    */
     // profile_idc, H.264-AVC-ISO_IEC_14496-10.pdf, page 45.
     u_int8_t        avc_profile; 
     // level_idc, H.264-AVC-ISO_IEC_14496-10.pdf, page 45.
     u_int8_t        avc_level; 
-    int            width;
-    int            height;
-    int            video_data_rate; // in bps
-    int            frame_rate;
-    int            duration;
     // lengthSizeMinusOne, H.264-AVC-ISO_IEC_14496-15.pdf, page 16
     int8_t          NAL_unit_length;
     u_int16_t       sequenceParameterSetLength;
@@ -214,9 +224,6 @@ public:
     /**
     * audio specified
     */
-    // @see: SrsCodecAudioType
-    int            audio_codec_id;
-    int            audio_data_rate; // in bps
     // 1.6.2.1 AudioSpecificConfig, in aac-mp4a-format-ISO_IEC_14496-3+2001.pdf, page 33.
     // audioObjectType, value defines in 7.1 Profiles, aac-iso-13818-7.pdf, page 40.
     u_int8_t        aac_profile;