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/services/network-filesystems/webdav-server-rs.nix

@@ -14,7 +14,7 @@ in
 {
   options = {
     services.webdav-server-rs = {
-      enable = mkEnableOption "WebDAV server";
+      enable = mkEnableOption (lib.mdDoc "WebDAV server");
 
       user = mkOption {
         type = types.str;