include: libctf: comment improvements

Describe a bit more clearly what effects a type being non-root-
visible has.  More consistently use the term non-root-visible
rather than hidden.  Document ctf_enum_iter.

include/
	* ctf-api.h (ctf_enum_iter): Document.
        (ctf_type_iter): Hidden, not non-root.  Mention that
        parent dictionaries are not traversed.