doc: Fix some grammar and inconsistencies

Michael Paquier <michael@paquier.xyz>

Commit: b3d89b7a889614a5ad549bae49b8a6fdda3a6bdd
Author: Michael Paquier <michael@paquier.xyz>
Date: 2020-09-10T06:50:42Z
Releases: 13.0
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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