doc: Reword restriction on partition keys in unique indexes

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: 63cbee6a786028790abc6ae9b9606366bae5e01d
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2018-03-12T16:32:28Z
Releases: 11.0
doc: Reword restriction on partition keys in unique indexes

New wording from David G. Johnston, who noticed the unreadable original
also.  Include his suggested test case as well.

Fix a typo I noticed elsewhere while doing this.

Discussion: https://postgr.es/m/CAKFQuwY4Ld7ecxL_KAmaxwt0FUu5VcPPN2L4dh+3BeYbrdBa5g@mail.gmail.com

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