nixos/nomad: describe the nomad cluster manager responsibilities

nixos/modules/services/networking/nomad.nix

@@ -66,6 +66,19 @@ in
         description = ''
           Configuration for Nomad. See the <link xlink:href="https://www.nomadproject.io/docs/configuration">documentation</link>
           for supported values.
+
+          Notes about `data_dir`:
+
+          If `data_dir` is set to a value other than the default value of
+          `"/var/lib/nomad"` it is the Nomad cluster manager's responsibility
+          to make sure that this directory exist and has the appropriate
+          permissions. One way to do this is with the `ExecStartPre` feature of
+          systemd.
+
+          Additionally, if `dropPrivileges` is `true` then `data_dir`
+          **cannot** be customized.  Setting `dropPrivileges` to `true` enables
+          the `DynamicUser` feature of systemd which directly manages and
+          operates on `StateDirectory`.
         '';
         example = literalExample ''
           {