Documentation: explain the usage of copyWith as a clone method

[rsevero]

dart_mappable copyWith generated methods can be used as clone tools but because of the overidding of the == operator, such a clone is recognized as equal to the original object when compared with the == operator. Only the identical() method detects the clone and the original as diferent objects.

I believe it would be helpful to users to include a section on the copyWith dart_mappable doc page explaing that. I suggest something like:

---

Clone

copyWith can be used as a clone tool when called without any parameters but be aware that the overriden == operator will detect the clone and the original as the same object. Only the identical() method detects that they are diferent objects.

Example:

@MappableClass()
class Foo with FooMappable {
  final int value;

  Foo(this.value);
}

void main() {
  var foo1 = Foo(1);
  var foo2 = foo1.copyWith();

  assert(foo1 == foo2); // succeeds
  assert(identical(foo1, foo2)); // fails
}