nixos/*: convert options with admonitions to MD

rendering changes only slightly, most changes are in spacing.

nixos/modules/virtualisation/virtualbox-host.nix

@@ -18,25 +18,25 @@ in
 
 {
   options.virtualisation.virtualbox.host = {
-    enable = mkEnableOption "VirtualBox" // {
-      description = ''
+    enable = mkEnableOption (lib.mdDoc "VirtualBox") // {
+      description = lib.mdDoc ''
         Whether to enable VirtualBox.
 
-        <note><para>
-          In order to pass USB devices from the host to the guests, the user
-          needs to be in the <literal>vboxusers</literal> group.
-        </para></note>
+        ::: {.note}
+        In order to pass USB devices from the host to the guests, the user
+        needs to be in the `vboxusers` group.
+        :::
       '';
     };
 
-    enableExtensionPack = mkEnableOption "VirtualBox extension pack" // {
-      description = ''
+    enableExtensionPack = mkEnableOption (lib.mdDoc "VirtualBox extension pack") // {
+      description = lib.mdDoc ''
         Whether to install the Oracle Extension Pack for VirtualBox.
 
-        <important><para>
-          You must set <literal>nixpkgs.config.allowUnfree = true</literal> in
-          order to use this.  This requires you accept the VirtualBox PUEL.
-        </para></important>
+        ::: {.important}
+        You must set `nixpkgs.config.allowUnfree = true` in
+        order to use this.  This requires you accept the VirtualBox PUEL.
+        :::
       '';
     };
 
@@ -60,15 +60,15 @@ in
     enableHardening = mkOption {
       type = types.bool;
       default = true;
-      description = ''
+      description = lib.mdDoc ''
         Enable hardened VirtualBox, which ensures that only the binaries in the
         system path get access to the devices exposed by the kernel modules
         instead of all users in the vboxusers group.
 
-        <important><para>
-          Disabling this can put your system's security at risk, as local users
-          in the vboxusers group can tamper with the VirtualBox device files.
-        </para></important>
+        ::: {.important}
+        Disabling this can put your system's security at risk, as local users
+        in the vboxusers group can tamper with the VirtualBox device files.
+        :::
       '';
     };