add rpc explanation

docs/protocol/serialization/signalproxy_rpc.md

 # SignalProxy RPC calls
 
+For RPC calls, the full name – e.g. "2createIdentity(Identity,QVariantMap)" has
+to be transmitted, including the number at the beginning and the arglist.
+
 ## C->S calls
 
 ### `2createIdentity(Identity,QVariantMap)`