Port some useful comments from newest version of David Hammerton's decoder...

Port some useful comments from newest version of David Hammerton's decoder (from which alac.c is based)

Originally committed as revision 9570 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/alac.c

@@ -86,8 +86,8 @@ typedef struct {
     uint8_t setinfo_rice_kmodifier; /* 0x0e */
     uint8_t setinfo_7f; /* 0x02 */
     uint16_t setinfo_80; /* 0x00ff */
-    uint32_t setinfo_82; /* 0x000020e7 */
-    uint32_t setinfo_86; /* 0x00069fe4 */
+    uint32_t setinfo_82; /* 0x000020e7 */ /* max sample size?? */
+    uint32_t setinfo_86; /* 0x00069fe4 */ /* bit rate (average)?? */
     uint32_t setinfo_8a_rate; /* 0x0000ac44 */
     /* end setinfo stuff */