doc: Simplify mention of unique indexes for NULL control

Bruce Momjian <bruce@momjian.us>

Commit: c20fd088f1c5fb5e492f40d427c45b8de38c879c
Author: Bruce Momjian <bruce@momjian.us>
Date: 2020-05-21T23:49:30Z
Releases: 13.0
doc:  Simplify mention of unique indexes for NULL control

Discussion: https://postgr.es/m/2304.1586532634@sss.pgh.pa.us

Backpatch-through: 9.5

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doc/src/sgml/indices.sgml modified +3 −19

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