Surface whether or not a getter can be overridden

[nex3]

In strong mode, a getter may not be overridden if it's defined as a field. For example,

class Super {
  final foo;
  get bar;
}

class Sub extends Super {
  get foo => 12;
  get bar => 13
}

Sub.foo is invalid, whereas Sub.bar is valid. This affects the public API of a class, so it should be mentioned in the documentation. For forwards-compatibility with future explicit member- or class-level control of virtual-ness, it would probably be good to document this as "this getter cannot be overidden in subclasses" or something like that.

cc @munificent @vsmenon

[sigurdm]

We could replace the "getter/setter pair" box with one that says "field"