Revamp fields documentation

Revamp fields documentation

• Actually give an example of a typed lexical.
• Also show the syntax parenthetically in the pod, since the phrase
 ‘typed lexical’ is not exactly well known.
• Don’t use dev version numbers.
• Don’t talk about pseudo-hashes so pervasively, as the fact that they
  are used is more an implementation detail now than a feature (except
  for phash).
• If we talk about restricted hashes, we should link to Hash::Util.
• -w is not the only way to turn warnings on.

Also mention in perlfunc that ‘my __PACKAGE__’ and ‘my CONSTANT’ work,
so that the only mentions are not in perl561delta and Lexical::Types.