net/rpc: Add documentation for client.Close

Fixes #16678

Change-Id: I48c2825d4fef55a75d2f99640a7079c56fce39db
Reviewed-on: https://go-review.googlesource.com/28370
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/rpc/client.go

@@ -274,6 +274,8 @@ func Dial(network, address string) (*Client, error) {
 	return NewClient(conn), nil
 }
 
+// Close calls the underlying codec's Close method. If the connection is already
+// shutting down, ErrShutdown is returned.
 func (client *Client) Close() error {
 	client.mutex.Lock()
 	if client.closing {