mgcp_sdp: add spec reference

Change-Id: I16cd453aa986cf9efa0716257230aed8739593da
diff --git a/src/libosmo-mgcp/mgcp_sdp.c b/src/libosmo-mgcp/mgcp_sdp.c
index ad2da7f..46b1dba 100644
--- a/src/libosmo-mgcp/mgcp_sdp.c
+++ b/src/libosmo-mgcp/mgcp_sdp.c
@@ -239,7 +239,7 @@
 		if (delimiter == ';' || delimiter == ',')
 			str_ptr[strlen(str_ptr) - 1] = '\0';
 
-		/* AMR octet aligned parameter */
+		/* AMR octet aligned parameter (see also RFC 3267, section 8.3) */
 		if (sscanf(param_str, "octet-align=%d", &amr_octet_aligned) == 1) {
 			fmtp_param->param.amr_octet_aligned_present = true;
 			fmtp_param->param.amr_octet_aligned = false;