pgsql: doc: add examples of creative use of unique expression indexes

doc: add examples of creative use of unique expression indexes

Unique expression indexes can constrain data in creative ways, so show
two examples.

Reported-by: Tuomas Leikola

Discussion: /messages/by-id/156760275564.1127.12321702656456074572@wrigleys.postgresql.org

Backpatch-through: 9.4

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2034bcc845f228c675c218e1928cf5337e74381d

Modified Files
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doc/src/sgml/indices.sgml | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)