Add example of destructuring assignment in parameter list to documentation

CoffeeScript is amazing.  I just found out that it supports destructuring assignment even inside the parameter list of a function.  It's a great feature and so I thought it'd be good to call it out in the documentation.

It looks like there was some discussion about this in class constructors in #1607 and #2378 and it just needed to get added.  So here's a pull request for documenting its use in a plain function, rather than the more complicated of a class method.

Thanks!