docs: document support for resumability and redelivery (#606)

Add missing documentation on resumability and redelivery in the
streamable transport.

Indirectly, also update stale transcluded examples. Example changes
weren't causing CI failure as we only trigger the docs check on changes
to the raw docs.

docs/client.md

@@ -53,9 +53,11 @@ func Example_roots() {
 
 	// Connect the server and client...
 	t1, t2 := mcp.NewInMemoryTransports()
-	if _, err := s.Connect(ctx, t1, nil); err != nil {
+	serverSession, err := s.Connect(ctx, t1, nil)
+	if err != nil {
 		log.Fatal(err)
 	}
+	defer serverSession.Close()
 
 	clientSession, err := c.Connect(ctx, t2, nil)
 	if err != nil {