treewide: convert all option docs to Markdown

This process was automated by [my fork of `nix-doc-munge`]. All
conversions were automatically checked to produce the same DocBook
result when converted back, modulo minor typographical/formatting
differences on the acceptable-to-desirable spectrum.

To reproduce this commit, run:

  $ NIX_PATH=nixpkgs=flake:nixpkgs/e7e69199f0372364a6106a1e735f68604f4c5a25 \
    nix shell nixpkgs#coreutils \
    -c find . -name '*.nix' \
    -exec nix run -- github:emilazy/nix-doc-munge/98dadf1f77351c2ba5dcb709a2a171d655f15099 \
    {} +
  $ ./format

[my fork of `nix-doc-munge`]: https://github.com/emilazy/nix-doc-munge/tree/home-manager

modules/services/fluidsynth.nix

@@ -11,12 +11,12 @@ in {
 
   options = {
     services.fluidsynth = {
-      enable = mkEnableOption "fluidsynth midi synthesizer";
+      enable = mkEnableOption (lib.mdDoc "fluidsynth midi synthesizer");
 
       soundFont = mkOption {
         type = types.path;
         default = "${pkgs.soundfont-fluid}/share/soundfonts/FluidR3_GM2-2.sf2";
-        description = ''
+        description = lib.mdDoc ''
           The soundfont file to use, in SoundFont 2 format.
         '';
       };
@@ -25,7 +25,7 @@ in {
         type = types.enum [ "jack" "pipewire-pulse" "pulseaudio" ];
         default = "pulseaudio";
         example = "pipewire-pulse";
-        description = ''
+        description = lib.mdDoc ''
           The systemd sound service to depend on.
         '';
       };
@@ -34,12 +34,9 @@ in {
         type = types.listOf types.str;
         default = [ ];
         example = [ "--sample-rate 96000" ];
-        description = ''
+        description = lib.mdDoc ''
           Extra arguments, added verbatim to the fluidsynth command. See
-          <citerefentry>
-            <refentrytitle>fluidsynth.conf</refentrytitle>
-            <manvolnum>1</manvolnum>
-          </citerefentry>.
+          {manpage}`fluidsynth.conf(1)`.
         '';
       };
     };