improved deprecation documentation and added warning when using snapshot with PyMongo3
improved deprecation documentation and added warning when using snapshot with PyMongo3

blaggacao/mongoengine,RussellLuo/mongoengine,Davidrjx/mongoengine,MongoEngine/mongoengine,prijindal/mongoengine,touilleMan/mongoengine,MakerReduxCorp/mongoengine,cogniteev/mongoengine,khallaghi/mongoengine,gmacon/mongoengine,DavidBord/mongoengine,nicolascine/mongoengine,reallistic/mongoengine,iandd0824/mongoengine,larsbutler/mongoengine,cogniteev/mongoengine,eli-b/mongoengine,9nix00/mongoengine,Catstyle/mongoengine,ConsumerPhysics/mongoengine,Davidrjx/mongoengine,MRigal/mongoengine,losintikfos/mongoengine,nathanial/mongoengine,gavincabbage/mongoengine,muhammad-ammar/mongoengine,ProgressivePlanning/mongoengine,ms5/mongoengine,yarneo/mongoengine,WingGao/mongoengine,jimmyshen/mongoengine,MakerReduxCorp/mongoengine,starsirius/mongoengine,sourcelair/mongoengine