rpc: add a note to RPC clients about thread-safeness

pkg/rpc/client/client.go

@@ -28,7 +28,8 @@ const (
 )
 
 // Client represents the middleman for executing JSON RPC calls
-// to remote NEO RPC nodes.
+// to remote NEO RPC nodes. Client is thread-safe and can be used from
+// multiple goroutines.
 type Client struct {
 	cli      *http.Client
 	endpoint *url.URL

pkg/rpc/client/wsclient.go

@@ -22,6 +22,8 @@ import (
 // servers. It's supposed to be faster than Client because it has persistent
 // connection to the server and at the same time is exposes some functionality
 // that is only provided via websockets (like event subscription mechanism).
+// WSClient is thread-safe and can be used from multiple goroutines to perform
+// RPC requests.
 type WSClient struct {
 	Client
 	// Notifications is a channel that is used to send events received from