net/rpc: `(*Client).Go` is not asynchronous with respect to outbound traffic

[tamird]

In the event of a network partition, net/rpc.(*Client).Go can block, which is surprising given the documented behaviour of this method.

[meirf]

As bdarnell writes in the linked cockroachdb issue:

net/rpc.Client.Go() is asynchronous in that it doesn't wait for a response, but it performs the write synchronously (via Client.send and codec.WriteRequest).

This isn't what someone would expect from the documentation: "Go invokes the function asynchronously." Even in the unlikely case where the authors of net/rpc didn't intend any blocking, making writing non blocking is probably too big of a change/too much of a feature and would violate the net/rpc freeze. Therefore it seems necessary and sufficient to just update the documentation on what is sync and what is async about Client.Go.