Documentation improvement patch

Dear all,

I have prepared a patch containing some minor inconsistencies in the 
documentation. Please, take a look.

The inconsistencies were noticed by: Ekaterina Kiryanova, Elena 
Indrupskaya, Maxim Yablokov, Anna Uraskova, Elena Karavaeva, and me.

We will be looking forward to your feedback.

The patch shall be applied to the REL_17_STABLE branch.

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Regards,
Oleg Sibiryakov
Technical Writer
Postgres Professional, The Russian Postgres Company
https://postgrespro.ru