pgsql: docs: improve 'capitals' inheritance example

docs: improve 'capitals' inheritance example

Adds constraints and improves wording.

Reported-by: 2552891@gmail.com

Discussion: /messages/by-id/159586122762.680.1361378513036616007@wrigleys.postgresql.org

Backpatch-through: 9.5

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/47c427d006267d752fee4655543ec99dabe9e61d

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