documentation: mention DialContext is non-blocking by default (#1970)

clientconn.go

@@ -417,10 +417,18 @@ func Dial(target string, opts ...DialOption) (*ClientConn, error) {
 	return DialContext(context.Background(), target, opts...)
 }
 
-// DialContext creates a client connection to the given target. ctx can be used to
-// cancel or expire the pending connection. Once this function returns, the
-// cancellation and expiration of ctx will be noop. Users should call ClientConn.Close
-// to terminate all the pending operations after this function returns.
+// DialContext creates a client connection to the given target. By default, it's
+// a non-blocking dial (the function won't wait for connections to be
+// established, and connecting happens in the background). To make it a blocking
+// dial, use WithBlock() dial option.
+//
+// In the non-blocking case, the ctx does not act against the connection. It
+// only controls the setup steps.
+//
+// In the blocking case, ctx can be used to cancel or expire the pending
+// connection. Once this function returns, the cancellation and expiration of
+// ctx will be noop. Users should call ClientConn.Close to terminate all the
+// pending operations after this function returns.
 //
 // The target name syntax is defined in
 // https://github.com/grpc/grpc/blob/master/doc/naming.md.