Clang AttributeReference: emit entries for "Undocumented" attributes.
Almost all attributes currently marked `Undocumented` are user-facing
attributes which _ought_ to be documented, but nobody has written it
yet. This change ensures that we at least acknowledge that these
attributes exist in the documentation, even if we have no description
of their semantics.
A new category, `InternalOnly` has been added for those few attributes
which are not user-facing, and should remain omitted from the docs.