Fix typos and grammar in documentation and code comments

Michael Paquier <michael@paquier.xyz>

Commit: dc6d285c2ef997f3a8d4c9f0e0599b62b0cbd054
Author: Michael Paquier <michael@paquier.xyz>
Date: 2021-04-09T04:53:17Z
Releases: 13.3
Fix typos and grammar in documentation and code comments

Comment fixes are applied on HEAD, and documentation improvements are
applied on back-branches where needed.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20210408164008.GJ6592@telsasoft.com
Backpatch-through: 9.6

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Documentation touched

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