nixos/gitlab: Document the restriction introduced on statePath

The state path now, since the transition from initialization in
preStart to using systemd-tmpfiles, has the following restriction: no
parent directory can be owned by any other user than root or the user
specified in services.gitlab.user. This is a potentially breaking
change and the cause of the error isn't immediately obvious, so
document it both in the release notes and statePath description.

nixos/doc/manual/release-notes/rl-1909.xml

@@ -505,6 +505,13 @@
        becomes <literal>services.gitlab.databasePasswordFile = "/path/to/secret_file"</literal> where the
        file <literal>secret_file</literal> contains the string <literal>supersecurepassword</literal>.
      </para>
+     <para>
+       The state path (<option>services.gitlab.statePath</option>) now has the following restriction:
+       no parent directory can be owned by any other user than <literal>root</literal> or the user
+       specified in <option>services.gitlab.user</option>; i.e. if <option>services.gitlab.statePath</option>
+       is set to <literal>/var/lib/gitlab/state</literal>, <literal>gitlab</literal> and all parent directories
+       must be owned by either <literal>root</literal> or the user specified in <option>services.gitlab.user</option>.
+     </para>
    </listitem>
   </itemizedlist>
  </section>

nixos/modules/services/misc/gitlab.nix

@@ -223,7 +223,15 @@ in {
       statePath = mkOption {
         type = types.str;
         default = "/var/gitlab/state";
-        description = "Gitlab state directory, logs are stored here.";
+        description = ''
+          Gitlab state directory. Configuration, repositories and
+          logs, among other things, are stored here.
+
+          The directory will be created automatically if it doesn't
+          exist already. Its parent directories must be owned by
+          either <literal>root</literal> or the user set in
+          <option>services.gitlab.user</option>.
+        '';
       };
 
       backupPath = mkOption {