Doc: document that we expect CHECK constraint conditions to be immutable.

This restriction is implicit in the check-only-once implementation we use
for table and domain constraints, but it wasn't spelled out anywhere, nor
was there any advice about how to alter a constraint's behavior safely.
Improve that.

I was also dissatisfied with the documentation of ALTER DOMAIN VALIDATE
CONSTRAINT, which entirely failed to explain the use of that feature; and
thence decided that ALTER TABLE VALIDATE CONSTRAINT could be documented
better as well.

Perhaps we should back-patch this, along with the related commit 36d442a25,
but for now I refrained.

Discussion: https://postgr.es/m/12539.1544107316@sss.pgh.pa.us

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