kernel-doc: make highlights more homogenous for the various backends

$type_struct_full and friends are only used by the restructuredText
backend, because it needs to separate enum/struct/typedef/union from
the name of the type.  However, $type_struct is *also* used by the rST
backend.  This is confusing.

This patch replaces $type_struct's use in the rST backend with a new
$type_fallback; it modifies $type_struct so that it can be used in the
rST backend; and creates regular expressions like $type_struct
for enum/typedef/union, for use in all backends.

Note that, compared to $type_*_full, in the new regexes $1 includes both
the "kind" and the name (before, $1 was pretty much a constant).

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