clientv3: document context.Context parameter to streaming APIs

[gyuho]

Related to #8980.

We haven't documented how our stream APIs work with client's contexts.

https://github.com/coreos/etcd/blob/e3da56a8dfb88c7ee8f6c282c986fb5154700910/clientv3/watch.go#L44-L53

https://github.com/coreos/etcd/blob/e3da56a8dfb88c7ee8f6c282c986fb5154700910/clientv3/lease.go#L116-L117

https://github.com/coreos/etcd/blob/e3da56a8dfb88c7ee8f6c282c986fb5154700910/clientv3/maintenance.go#L57-L58

Would cancelling the context close the watch stream? What if the context timeouts? If the stream is kept open, what is the retry policy? What happens to the underlying io.ReadCloser when the context gets cancelled?

Also

https://github.com/coreos/etcd/blob/b48cf77abb78755c8654eb8da61bc90a32802e7d/clientv3/concurrency/session.go#L132-L141

[gyuho]

/cc @fanminshi

Another reference: d59f6c1

[gyuho]

WithContext is already documented well. Closing!

[xiang90]

@gyuho good job on this one!!