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