[appledoc] Include subclassing header.

Use `--no-merge-categories` to prevent subclassing-only documentation
from appearing in the general API reference.  Use consistent category
names.
[appledoc] Include ASRangeController.
[appledoc] Enable explicit crossrefs.

Fix warnings for existing `@see ...` crossrefs.
[appledoc] Fix all appledoc warnings.  (#65)