docs: don't recommend sudo to run the migration script

The migration script tries switching EUID by itself and will error out
with a recommendation to try sudo if it cannot.

(cherry picked from commit a38e14460f7b35fb7ef3f0e4ced87d89754c87f9)

docs/migrations.rst

@@ -59,13 +59,16 @@ For remediating this issue the following steps are required:
    - ``--layout default`` unless ``useFSLayout`` is enabled, then ``--layout folder``
    - The value of ``mailserver.mailDirectory``, which defaults to ``/var/vmail``
 
+   The script should be run under the user who owns the ``mailDirectory``.
+   If run as root it will try to switch into the appropriate user by itself.
+
    The script will not modify your data unless called with ``--execute``.
 
    Example:
 
    .. code-block:: bash
 
-      sudo -u virtualMail ./nixos-mailserver-migration-03.py --layout default /var/vmail
+      ./nixos-mailserver-migration-03.py --layout default /var/vmail
 
 5. Review the commands. They should be