scripts/kernel-doc: handle DECLARE_HASHTABLE

DECLARE_HASHTABLE needs similar handling to DECLARE_BITMAP
because otherwise kernel-doc assumes the member name is the
second, not first macro parameter.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>