add comments of audio profile supported by srs.

@@ -802,6 +802,7 @@ vhost example.transcode.srs.com {
             # other ffmpeg audio params
             aparams {
                 # audio params, @see: http://ffmpeg.org/ffmpeg-codecs.html#Audio-Encoders
+                # @remark SRS supported aac profile for HLS is: aac_low, aac_he, aac_he_v2
                 profile:a   aac_low;
             }
             # output format, can be: