MAN: Document that auth and access IPA and AD providers rely on id_provider being set to the same type
    
    The IPA and AD auth and access providers rely (often for performance
    reasons) on certain properties that are set during the user or
    group resolution with the same provider type.
    
    However, there are users who wish to combine different provider types,
    typically to use identities from local UNIX files but authenticate
    against a remote server. We should discourage that in our documentation
    (but at the same time, I think flat our failing would be too harsh..)
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3547
    
    Reviewed-by: Justin Stephenson <jstephen@redhat.com>