Fix Trac #16391 by using occCheckExpand in TcValidity
The type-variables-escaping-their-scope-via-kinds check in
TcValidity was failing to properly expand type synonyms, which led
to Trac #16391. This is easily fixed by using occCheckExpand before
performing the validity check.
Along the way, I refactored this check out into its own function,
and sprinkled references to Notes to better explain all of the moving
parts. Many thanks to @simonpj for the suggestions.
Bumps the haddock submodule.