Revise @typedef, enforce namespaced singularity/uniqueness

This PR implements https://github.com/phpDocumentor/fig-standards/pull/54#issuecomment-52316149

Please keep the discussion on #54 and/or #7.
Case (3) was:
> You consume a library with magic methods who does not implement the `@method`
   tag but still want to document which methods are on it yourself.

Happy to add it back, but IMO this example use-case was an exact definition for why `@typedef` is problematic:

Instead of adjusting and improving documentation upstream, your entire downstream code starts to type-hint a custom pseudo-type all over the place instead.

That may be an extreme edge-case (if upstream is unwilling to fix their docs), but that's not how things are supposed to work, and thus a negative example.