Following a long conversation with Richard, this patch tidies up the
handling of return kinds for data/newtype declarations (vanilla,
family, and instance).

I have substantially edited the Notes in TyCl, so they would
bear careful reading.

Fixes #18300.

In GHC.Tc.Instance.Family.newFamInst we were checking some
Lint-like properties with ASSSERT.  Instead I've added
Lint.lintAxiom, and called it from newFamInst.

The one new test, T18300, causes an ASSERT failure in HEAD.