Summary:
This is a follow-up to D5173, which permitted
unparenthesized kind signatures in certain places. One place that
appeared to be overlooked was the right-hand sides of type synonyms,
which this patch addresses by introducing a `ktypedoc` parser
production (which is to `ctypdoc` as `ktype` is to `ctype`) and
using it in the right place.

Test Plan: make test TEST="KindSigs T15781"

Reviewers: harpocrates, bgamari

Reviewed By: harpocrates

Subscribers: rwbarton, mpickering, carter

GHC Trac Issues: #15781

Differential Revision: https://phabricator.haskell.org/D5245