docs: Add a section on SELinux modules to the HSM design
    
    Additional SELinux rules are necessary for the HSM to be
    managed by IPA and certmonger. Given the infinite possible
    naming combinations of library paths and modules this is
    a best effort. A message is logged if a missing module
    is detected.
    
    Related: https://pagure.io/freeipa/issue/9273
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>