better comments to clarify use of code and sub_code

Author: MarcelOliver

osi_trafficsign.proto

@@ -300,21 +300,25 @@ message TrafficSign
             optional bool is_out_of_service = 7;
 
             // Specifies the country. See ISO 3166-1, alpha-2 codes
-            // https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2
-            // e.g. "DE", "FR", "AT
-            //"
+            // https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2. <br>
+            // e.g. "DE", "FR", "AT"
+            //
             optional string country = 8;
 
-            // The year the traffic rules come into force
+            // The year the traffic rules come into force. <br>
             // e.g. "2017"
             //
             optional string country_revision = 9;
 
-            // Code identifier according to country code
+            // Code identifier according to country code. <br>
+            // code is only unique in combination with #country and #country_revision.  <br>
+            // e.g. http://www.vzkat.de/2017/VzKat.htm
             //
             optional string code = 10;
 
-            // Subcode identifier according to country code
+            // Subcode identifier according to country code. <br>
+            // sub_code is only unique in combination with #country and #country_revision.  <br>
+            // e.g. http://www.vzkat.de/2017/VzKat.htm
             //
             optional string sub_code = 11;
 
@@ -5586,17 +5590,25 @@ message TrafficSign
             optional bool is_out_of_service = 7;
 
             // Specifies the country. See ISO 3166-1, alpha-2 codes
-            // https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2
+            // https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2.  <br>
+            // e.g. "DE", "FR", "AT"
+            //
             optional string country = 8;
 
-            // The year the traffic rules come into force
+            // The year the traffic rules come into force. <br>
+            // e.g. "2017"
+            //
             optional string country_revision = 9;
 
-            // Code identifier according to country code
+            // Code identifier according to country code. <br>
+            // code is only unique in combination with #country and #country_revision.  <br>
+            // e.g. http://www.vzkat.de/2017/VzKat.htm
             //
             optional string code = 10;
 
-            // Subcode identifier according to country code
+            // Subcode identifier according to country code. <br>
+            // sub_code is only unique in combination with #country and #country_revision.  <br>
+            // e.g. http://www.vzkat.de/2017/VzKat.htm
             //
             optional string sub_code = 11;