Enum-shorthand: Fix constant pattern.

Fix constant pattern, clean-up, and expansion of the constant section.

* Changes grammar for constant pattern to not allow full selector chain,
  only shorthands for `T.id` and `const T.(id|new)(args)`.
  so the shorthand doesn't accept more than the existing limited productions.
* No other grammatical or semantic changes intended.
* Expand the "Constants" section.
* Mention explicitly that a context type scheme is a semantic type,
  so it cannot refer to a type alias.