treewide: automatically md-convert option descriptions

the conversion procedure is simple:

 - find all things that look like options, ie calls to either `mkOption`
   or `lib.mkOption` that take an attrset. remember the attrset as the
   option
 - for all options, find a `description` attribute who's value is not a
   call to `mdDoc` or `lib.mdDoc`
 - textually convert the entire value of the attribute to MD with a few
   simple regexes (the set from mdize-module.sh)
 - if the change produced a change in the manual output, discard
 - if the change kept the manual unchanged, add some text to the
   description to make sure we've actually found an option. if the
   manual changes this time, keep the converted description

this procedure converts 80% of nixos options to markdown. around 2000
options remain to be inspected, but most of those fail the "does not
change the manual output check": currently the MD conversion process
does not faithfully convert docbook tags like <code> and <package>, so
any option using such tags will not be converted at all.

nixos/modules/services/computing/foldingathome/client.nix

@@ -24,7 +24,7 @@ in
       type = types.package;
       default = pkgs.fahclient;
       defaultText = literalExpression "pkgs.fahclient";
-      description = ''
+      description = lib.mdDoc ''
         Which Folding@home client to use.
       '';
     };
@@ -32,7 +32,7 @@ in
     user = mkOption {
       type = types.nullOr types.str;
       default = null;
-      description = ''
+      description = lib.mdDoc ''
         The user associated with the reported computation results. This will
         be used in the ranking statistics.
       '';
@@ -41,7 +41,7 @@ in
     team = mkOption {
       type = types.int;
       default = 236565;
-      description = ''
+      description = lib.mdDoc ''
         The team ID associated with the reported computation results. This
         will be used in the ranking statistics.
 
@@ -52,7 +52,7 @@ in
     daemonNiceLevel = mkOption {
       type = types.ints.between (-20) 19;
       default = 0;
-      description = ''
+      description = lib.mdDoc ''
         Daemon process priority for FAHClient.
         0 is the default Unix process priority, 19 is the lowest.
       '';
@@ -61,9 +61,9 @@ in
     extraArgs = mkOption {
       type = types.listOf types.str;
       default = [];
-      description = ''
+      description = lib.mdDoc ''
         Extra startup options for the FAHClient. Run
-        <literal>FAHClient --help</literal> to find all the available options.
+        `FAHClient --help` to find all the available options.
       '';
     };
   };