[client-shell] remind user that an API method expects a payload

[djmitche]

It's easy to type something like

$ taskcluster api workerManager createWorker somePool someGroup someWorker

and then let it sit there, thinking it's hung. It's not, it's just waiting for the API method payload on stdin.

In cases where stdin is a tty, let's help the user by printing "Waiting for API method payload on stdin" to stderr.