pgsql: 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(at)highgo(dot)com>
Reviewed-by: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Discussion: https://postgr.es/m/E74C57FA-1DD0-4C8E-8FB1-538034752592@gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d16be8605f5f699020fe4e6eaef008a41407a9bb

Modified Files
--------------
doc/src/sgml/ddl.sgml | 33 +++++++++++++++++++--------------
1 file changed, 19 insertions(+), 14 deletions(-)