Fix some typos, grammar and style in docs and comments

Michael Paquier <michael@paquier.xyz>

Commit: 70dc2385f9a20c8d397a244b5237831790d8e2a6
Author: Michael Paquier <michael@paquier.xyz>
Date: 2021-02-24T07:14:03Z
Releases: 11.12
Fix some typos, grammar and style in docs and comments

The portions fixing the documentation are backpatched where needed.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20210210235557.GQ20012@telsasoft.com
backpatch-through: 9.6

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Documentation touched

Discussion