reword MPEG1/2/3 audio to refer to Element Stream of ISO specs

Unfortunately these (ancient) specs are nowhere to be found freely.

codec_specs.md

@@ -707,7 +707,7 @@ Codec ID: A_MPEG/L1
 
 Codec Name: MPEG Audio 1, 2 Layer I
 
-Description: The data contain everything needed for playback in the MPEG Audio header of each frame. Corresponding ACM wFormatTag : 0x0050
+Description: Frames corresponds to Audio Frames of a Layer I bitstream as defined in [@!ISO.11172-3]. Corresponding ACM wFormatTag : 0x0050
 
 Initialization: none
 
@@ -717,7 +717,7 @@ Codec ID: A_MPEG/L2
 
 Codec Name: MPEG Audio 1, 2 Layer II
 
-Description: The data contain everything needed for playback in the MPEG Audio header of each frame. Corresponding ACM wFormatTag : 0x0050
+Description: Frames corresponds to Audio Frames of a Layer II bitstream as defined in [@!ISO.11172-3]. Corresponding ACM wFormatTag : 0x0050
 
 Initialization: none
 
@@ -727,7 +727,7 @@ Codec ID: A_MPEG/L3
 
 Codec Name: MPEG Audio 1, 2, 2.5 Layer III
 
-Description: The data contain everything needed for playback in the MPEG Audio header of each frame. Corresponding ACM wFormatTag : 0x0055
+Description: Frames corresponds to Audio Frames of a Layer III bitstream as defined in [@!ISO.11172-3]. Corresponding ACM wFormatTag : 0x0055
 
 Initialization: none
 

rfc_backmatter_codec.md

@@ -183,6 +183,17 @@
   <seriesInfo name="ISO" value="11172-2:1993" />
 </reference>
 
+<reference anchor="ISO.11172-3">
+  <front>
+    <title>Information technology — Coding of moving pictures and associated audio for digital storage median at up to about 1,5 Mbit/s - Part 3: Audio</title>
+    <author>
+      <organization>International Organization for Standardization</organization>
+    </author>
+    <date month="August" year="1993"/>
+  </front>
+  <seriesInfo name="ISO" value="11172-2:1993" />
+</reference>
+
 <reference anchor="ISO.13818-2">
   <front>
     <title>Information technology — Generic coding of moving pictures and associated audio information - Part 2: Video</title>