pgsql: docs: improve 'capitals' inheritance example

docs:  improve 'capitals' inheritance example

Adds constraints and improves wording.

Reported-by: 2552891@gmail.com

Discussion: https://postgr.es/m/159586122762.680.1361378513036616007@wrigleys.postgresql.org

Backpatch-through: 9.5

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a76d83a00a9151dda2d1258100421ba493d6229a

Modified Files
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doc/src/sgml/advanced.sgml | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)