mgcp_client.h: also add spec ref to the other 3gpp defined payload types

Some payload type numbers are defined in 3GPP TS 48.103, let's put the
spec reference between all of those so that it is immediately clear
where those numbers come from.

Change-Id: Ie9d949ee72286ee4de7590c99acc740011208466
diff --git a/include/osmocom/mgcp_client/mgcp_client.h b/include/osmocom/mgcp_client/mgcp_client.h
index 0fb5dee..6adaf4b 100644
--- a/include/osmocom/mgcp_client/mgcp_client.h
+++ b/include/osmocom/mgcp_client/mgcp_client.h
@@ -45,12 +45,12 @@
 	CODEC_GSM_8000_1 = 3,
 	CODEC_PCMA_8000_1 = 8,
 	CODEC_G729_8000_1 = 18,
-	CODEC_GSMEFR_8000_1 = 110,
-	CODEC_GSMHR_8000_1 = 111,
-	CODEC_AMR_8000_1 = 112,
-	CODEC_AMRWB_16000_1 = 113,
+	CODEC_GSMEFR_8000_1 = 110,	/* 3GPP TS 48.103 table 5.4.2.2.1 */
+	CODEC_GSMHR_8000_1 = 111,	/* 3GPP TS 48.103 table 5.4.2.2.1 */
+	CODEC_AMR_8000_1 = 112,		/* 3GPP TS 48.103 table 5.4.2.2.1 */
+	CODEC_AMRWB_16000_1 = 113,	/* 3GPP TS 48.103 table 5.4.2.2.1 */
 	CODEC_IUFP = 96,
-	CODEC_CLEARMODE = 120, /* 3GPP TS 48.103 table 5.4.2.2.1 */
+	CODEC_CLEARMODE = 120,		/* 3GPP TS 48.103 table 5.4.2.2.1 */
 };
 /* Note: when new codec types are added, the corresponding value strings
  * in mgcp_client.c (codec_table) must be updated as well. Enumerations