treewide: adjust some DocBook for conversion

The NixOS variant of Markdown doesn't make a distinction between
`<code>` and `<literal>` or `<quote>` and... quotes, and doesn't
support `<parameter>` or `<replaceable>`. These are infrequently used
(apart from `<code>`) and don't add much, so just convert them to
simpler forms to allow the options containing them to be converted
to Markdown automatically.

A few minor syntactic adjustments were also made to make
`nix-doc-munge`'s job easier.

modules/programs/bash.nix

@@ -103,7 +103,7 @@ in {
         example = [ "extglob" "-cdspell" ];
         description = ''
           Shell options to set. Prefix an option with
-          <quote><literal>-</literal></quote> to unset.
+          "<literal>-</literal>" to unset.
         '';
       };