doc: Improve some markups and some wording around archiving modules

Michael Paquier <michael@paquier.xyz>

Commit: 31e121d4f7bb3aad7a26021cbc7cfb43a077c7e3
Author: Michael Paquier <michael@paquier.xyz>
Date: 2022-08-19T01:00:12Z
Releases: 16.0
doc: Improve some markups and some wording around archiving modules

This commit adds or fixes markups used in a couple of places in the docs
(for <command>, <systemitem> and <literal>).  While on it, this
clarifies some of the documentation added recently for archiving modules
with archive_command, that would still be used as default choice if no
external module is defined (though an archive module could as well use
an archive_command).

Author: Maxim Yablokov
Discussion: https://postgr.es/m/b47ec4e8-6f6a-2aba-038e-d5db150b245e@postgrespro.ru
Backpatch-through: 15

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