nixos/*: automatically convert option descriptions

conversions were done using https://github.com/pennae/nix-doc-munge
using (probably) rev f34e145 running

    nix-doc-munge nixos/**/*.nix
    nix-doc-munge --import nixos/**/*.nix

the tool ensures that only changes that could affect the generated
manual *but don't* are committed, other changes require manual review
and are discarded.

nixos/modules/system/boot/kernel.nix

@@ -48,17 +48,17 @@ in
       # - some of it might not even evaluate correctly.
       defaultText = literalExpression "pkgs.linuxPackages";
       example = literalExpression "pkgs.linuxKernel.packages.linux_5_10";
-      description = ''
+      description = lib.mdDoc ''
         This option allows you to override the Linux kernel used by
         NixOS.  Since things like external kernel module packages are
         tied to the kernel you're using, it also overrides those.
         This option is a function that takes Nixpkgs as an argument
         (as a convenience), and returns an attribute set containing at
-        the very least an attribute <varname>kernel</varname>.
+        the very least an attribute {var}`kernel`.
         Additional attributes may be needed depending on your
         configuration.  For instance, if you use the NVIDIA X driver,
         then it also needs to contain an attribute
-        <varname>nvidia_x11</varname>.
+        {var}`nvidia_x11`.
       '';
     };
 
@@ -73,9 +73,9 @@ in
       type = types.str;
       default = "";
       example = "my secret seed";
-      description = ''
-        Provides a custom seed for the <varname>RANDSTRUCT</varname> security
-        option of the Linux kernel. Note that <varname>RANDSTRUCT</varname> is
+      description = lib.mdDoc ''
+        Provides a custom seed for the {var}`RANDSTRUCT` security
+        option of the Linux kernel. Note that {var}`RANDSTRUCT` is
         only enabled in NixOS hardened kernels. Using a custom seed requires
         building the kernel and dependent packages locally, since this
         customization happens at build time.