doc: Fix some grammar and inconsistencies

Michael Paquier <michael@paquier.xyz>

Commit: 447c1cb2f67fd6941a41bc0ec2e5c7acf0759531
Author: Michael Paquier <michael@paquier.xyz>
Date: 2020-09-10T06:50:50Z
Releases: 11.10
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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