doc: Clarify inherited constraint behavior

doc: Clarify inherited constraint behavior

Update the table inheritance documentation to mention not-null constraints
alongside check constraints where inherited constraints are discussed.

Also clarify that some properties of inherited constraints can now be altered
directly on child tables, while the resulting constraint must remain compatible
with its inherited parent constraints.  For multiple inheritance, say explicitly
that when a column or constraint is inherited from more than one parent, the
stricter definition applies.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com>
Discussion: https://postgr.es/m/E74C57FA-1DD0-4C8E-8FB1-538034752592@gmail.com