MAN: Document that the secrets provider can only be specified in a per-client section
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3417
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Justin Stephenson <jstephen@redhat.com>