nixos/*: convert options with admonitions to MD

rendering changes only slightly, most changes are in spacing.

nixos/modules/services/web-servers/phpfpm/default.nix

@@ -40,9 +40,12 @@ let
         socket = mkOption {
           type = types.str;
           readOnly = true;
-          description = ''
+          description = lib.mdDoc ''
             Path to the unix socket file on which to accept FastCGI requests.
-            <note><para>This option is read-only and managed by NixOS.</para></note>
+
+            ::: {.note}
+            This option is read-only and managed by NixOS.
+            :::
           '';
           example = "${runtimeDir}/<name>.sock";
         };