doc: Add nsswitch.conf note to manpage
We want to add note about nsswitch.conf configuration
into sssd-files manpage.
Resolves:
https://pagure.io/SSSD/sssd/issue/3750
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
Reviewed-by: Justin Stephenson <jstephen@redhat.com>