docs: note that set shell is not used for [script]

[iloveitaly]

this was surprising functionality for me.

IMHO I think inheriting shell if script-interpreter is not set might be better

[casey]

Agreed it's confusing!

The reason for not using the value of set shell is that the argument to set shell is a string, whereas the argument to set script-interpreter is a path to a script. So set shell needs -c, and set script-interpreter doesn't.

[iloveitaly]

Yeah, definitely makes sense once you understand that [script] essentially generates an executable file with the contents of the recipe.