Can we add `stdin` and `stdout` to the docs?

[dancrumb]

Is your feature request related to a problem?

I had to dig around pretty deeply in the code to find out that I can indeed use STDERR as my output stream.

Describe the solution you'd like

Just an update to the docs explaining that stdin and stdout are permitted options for a prompt

Describe alternatives you've considered

The alternative is just to use an undocumented feature, which is always risky.

Additional context

Nope... just a small documentation update