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

doc/src/sgml/ref/psql-ref.sgml

@@ -4658,10 +4658,10 @@ testdb=> \set PROMPT1 '%[%033[1;33;40m%]%n@%/%R%[%033[0m%]%# '
     and SQL object names in many (by no means all) contexts.  For example,
     at the start of a command, typing <literal>ins</literal> and pressing
     TAB will fill in <literal>insert into </literal>.  Then, typing a few
-    characters of a table or schema name and pressing TAB will fill in the
-    unfinished name, or offer a menu of possible completions when there's
-    more than one.  (Depending on the library in use, you may need to
-    press TAB more than once to get a menu.)
+    characters of a table or schema name and pressing <literal>TAB</literal>
+    will fill in the unfinished name, or offer a menu of possible completions
+    when there's more than one.  (Depending on the library in use, you may need to
+    press <literal>TAB</literal> more than once to get a menu.)
     </para>
 
     <para>
@@ -4690,7 +4690,7 @@ $endif
      of <application>psql</application>.  This prevents tab completion,
      use or recording of command line history, and editing of multi-line
      commands.  It is particularly useful when you need to copy-and-paste
-     text that contains TAB characters.
+     text that contains <literal>TAB</literal> characters.
     </para>
    </refsect3>
   </refsect2>