Document support for keyword-only args

[piccolbo]

It's not possible to decorate functions with keyword only args, the reason being that it's not possible to define a signature with this property. But the no arg decorator works.

[piccolbo]

Not sure what the UI would be: param(keyword_only=TRUE)? Verbose and easy to create impossible sigs. The * syntax can not be used as it is special to defs.

[piccolbo]

This is actually supported with a kw_only arg. Sometimes my code is so concise I forget I implemented it! But it seems missing in the docs so I will keep this open with the goal of fixing that.