doc: Fix some grammar and inconsistencies

Michael Paquier <michael@paquier.xyz>

Commit: 25ff747721a5f4d386127528f50fd81d6cea365a
Author: Michael Paquier <michael@paquier.xyz>
Date: 2020-09-10T06:50:46Z
Releases: 12.5
doc: Fix some grammar and inconsistencies

Some comments are fixed while on it.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20200818171702.GK17022@telsasoft.com
Backpatch-through: 9.6

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