Improve documentation on Named Method Parameters

Section 3.2.2 of the Groovy Object Orientation documentation describes named method parameters (http://groovy-lang.org/objectorientation.html#_named_parameters_2).

All name parameters are collected in a map, and passed in as first parameter to the called method. One corner case is not mentioned, though: when you call the method without any named parameter, the map is not created and not passed in. Thus, one should make the map optional:

def foo(Map args=[:], Integer number)

If not optional, one gets a groovy.lang.MissingMethodException: No signature of method.

This is a tricky, unintuitive case which should be explicitly mentioned in the documentation.