Documentation missing on payload option

[nomorebugs]

I have to send the current token as payload in request body to refresh it.

It took me some time reading the source code to figure out, that a dynamic payload is possible by wrapping a Promise in a function.

This feature should be documented. Furthermore it appears a bit strange, that neither a simple callback function is used nor an observable directly (as rxjs is used in source) and this promise is converted into an observable later on.