NetworkPkg/IScsiDxe: Add the clarification compared to IScsiDxe in MdeModulePkg.
    
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1205
    
    This patch is to add the driver usage/difference clarification
    compared to IScsiDxe in MdeModulePkg.
    
    Cc: Ye Ting <ting.ye@intel.com>
    Cc: Fu Siyuan <siyuan.fu@intel.com>
    Cc: Laszlo Ersek <lersek@redhat.com>
    Contributed-under: TianoCore Contribution Agreement 1.1
    Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
    Reviewed-by: Laszlo Ersek <lersek@redhat.com>