Doc: Fix various typos

Michael Paquier <michael@paquier.xyz>

Commit: d38753c6f79d1eb7d3096ff777ec6df0bc7e5008
Author: Michael Paquier <michael@paquier.xyz>
Date: 2019-08-20T04:45:53Z
Releases: 12.0
Doc: Fix various typos

All those fixes are already included on HEAD thanks to for example
c96581a and 66bde49, and have gone missing on back-branches.

Author: Alexander Lakhin, Liudmila Mantrova
Discussion: https://postgr.es/m/CAEkD-mDJHV3bhgezu3MUafJLoAKsOOT86+wHukKU8_NeiJYhLQ@mail.gmail.com
Backpatch-through: 9.4

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