Ubuntu 20.04: Add a warning about disabling ACLs

docs/Getting Started/Ubuntu/Ubuntu 20.04 Root on ZFS.rst

@@ -383,6 +383,8 @@ Step 2: Disk Formatting
      ``-o acltype=posixacl`` (note: lowercase “o”) to the ``zfs create``
      for ``/var/log``, as `journald requires ACLs
      <https://askubuntu.com/questions/970886/journalctl-says-failed-to-search-journal-acl-operation-not-supported>`__
+     Also, `disabling ACLs apparently breaks umask handling with NFSv4
+     <https://bugs.launchpad.net/ubuntu/+source/nfs-utils/+bug/1779736>`__.
    - Setting ``normalization=formD`` eliminates some corner cases relating
      to UTF-8 filename normalization. It also implies ``utf8only=on``,
      which means that only UTF-8 filenames are allowed. If you care to