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.
The default value of `programs.ncmpcpp.mpdMusicDir` is taken from
`services.mpd.musicDirectory` if the mpd module is enabled, which has
type `either path str`. `programs.ncmpcpp.mpdMusicDir` did not accept
`str` values, though, so an error was raised when the default value was
used and `services.mpd.musicDirectory` was set to a value of type `str`.
This commit changes the type of `programs.ncmpcpp.mpdMusicDir` to also
accept `str` to reflect the type of `services.mpd.musicDirectory`.
Fixes#3560
