devguide: add watermark to sequence diagrams

Make it more evident that the sequence diagrams in the transactions
page are generated with Mscgen

doc/userguide/devguide/extending/app-layer/diagrams/DnsUnidirectionalTransactions.msc

@@ -13,4 +13,7 @@ msc {
     |||;
     b =>> a [ label = "DNS Response" ];
     --- [ label = "Transaction 2 Completed" ];
+
+    |||;
+    ||| [label="[ generated with Mscgen ]", textcolor="gray"];
 }

doc/userguide/devguide/extending/app-layer/diagrams/HTTP2BidirectionalTransaction.msc

@@ -13,6 +13,8 @@ msc {
     b =>> a [ label = "Response" ];
     |||;
     --- [ label = "Transaction Completed" ];
+    |||;
+    ||| [label="[ generated with Mscgen ]", textcolor="gray"];
 }
 
 # Reference: https://tools.ietf.org/html/rfc7540#section-8.1

doc/userguide/devguide/extending/app-layer/diagrams/TemplateTransaction.msc

@@ -12,4 +12,7 @@ msc {
     b =>> a [ label = "Response ('3:Bye')" ];
     |||;
     --- [ label = "Transaction Completed" ];
+
+    |||;
+    ||| [label="[ generated with Mscgen ]", textcolor="gray"];
 }

doc/userguide/devguide/extending/app-layer/diagrams/TlsHandshake.msc

@@ -28,4 +28,7 @@ msc {
     # TLS_STATE_FINISHED = 3
     a abox b [ label = "TLS_STATE_FINISHED" ];
     --- [ label = "Transaction Completed" ];
+
+    |||;
+    ||| [label="[ generated with Mscgen ]", textcolor="gray"];
 }