doc: Improve some markups and some wording around archiving modules

doc: Improve some markups and some wording around archiving modules

This commit adds or fixes used markups in a couple of places in the docs
(for <command>, <systemitem> and <literal>).  While on it, clarify 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