Describe named arguments constructor option for enum

Groovy enum includes no-arg and named-args constructors if no explicit constuctors are declared. Add description to language docs for named arguments constructor.

enum Foo {
  BAR(number:42, string:'x')
  final Number number
  public String string
}
print Foo.BAR.number // prints: 42
print Foo.BAR.string // prints: null