Add `@super` macro.

[ykmnkmi]

To add additional documentation in subclasses, I know the @template macro exists.

For example:

class A {
  /// Prints `a`.
  void log() {
    print('a');
  }
}

class B extends A {
  /// {@super} Then prints `b`.
  void log() {
    print('b');
  }
}

No new line after the macro call, so it shows: "Prints a. Then prints b."

[sigurdm]

interesting idea. @szakarias wdyt?