pgsql: Doc: clarify NULLS NOT DISTINCT use in unique indexes

Doc: clarify NULLS NOT DISTINCT use in unique indexes

indexes-unique.html mentioned nothing about the availability of NULLS NOT
DISTINCT to modify the NULLs-are-not-equal behavior of unique indexes.
Add this to the synopsis and clarify what it does regarding NULLs.

Author: David Gilman, David Rowley
Reviewed-by: Corey Huinker
Discussion: https://postgr.es/m/CALBH9DDr3NLqzWop1z5uZE-M5G_GYUuAeHFHQeyzFbNd8W0d=Q@mail.gmail.com
Backpatch-through: 15, where NULLS NOT DISTINCT was added

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0d0aeb04c1277edb0733cbf5bf4243c47a439015

Modified Files
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doc/src/sgml/indices.sgml | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)