Provide guidance for stack-threads configuration in async and reactive applications

[sdeleuze]

As discussed in #663, the default stack-threads value (300 if I am not mistaken) is perfectly valid for regular blocking applications, but not for fully async/reactive ones, where a lower value would more accurate.

Since that's not obvious, it would be nice if the documentation could provide some guidance to configure stack-threads for such kind of application, not very widely deployed yet, but likely to become more and more in the future.

[sdeleuze]

Thanks @nebhale.