pgsql: doc: Improve some markups and some wording around archiving modu

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/31e121d4f7bb3aad7a26021cbc7cfb43a077c7e3

Modified Files
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doc/src/sgml/adminpack.sgml | 7 ++++---
doc/src/sgml/archive-modules.sgml | 6 +++---
doc/src/sgml/backup.sgml | 23 +++++++++++++----------
doc/src/sgml/basebackup-to-shell.sgml | 8 ++++----
doc/src/sgml/ref/alter_publication.sgml | 2 +-
doc/src/sgml/ref/pg_basebackup.sgml | 4 +---
doc/src/sgml/ref/psql-ref.sgml | 10 +++++-----
7 files changed, 31 insertions(+), 29 deletions(-)