kernel-doc: use rst C domain directives and references for types

First, the headings for structs, enums and typedefs will be similar to
functions. Second, this provides a kind of namespace for cross
references. Third, and most importantly, the return and parameter types
from .. c:function:: definitions will automagically become cross
references to the documented types.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>