Update defining-documents.rst
Update defining-documents.rst

Add primary_key notice according to issue #985
blaggacao/mongoengine,MRigal/mongoengine,gmacon/mongoengine,reallistic/mongoengine,khallaghi/mongoengine,ms5/mongoengine,starsirius/mongoengine,prijindal/mongoengine,yarneo/mongoengine,RussellLuo/mongoengine,Davidrjx/mongoengine,nathanial/mongoengine,ConsumerPhysics/mongoengine,gavincabbage/mongoengine,WingGao/mongoengine,Davidrjx/mongoengine,jimmyshen/mongoengine,cogniteev/mongoengine,cogniteev/mongoengine,DavidBord/mongoengine,muhammad-ammar/mongoengine,sourcelair/mongoengine,MakerReduxCorp/mongoengine,9nix00/mongoengine,losintikfos/mongoengine,iandd0824/mongoengine,touilleMan/mongoengine,MongoEngine/mongoengine,nicolascine/mongoengine,MakerReduxCorp/mongoengine,ProgressivePlanning/mongoengine,Catstyle/mongoengine,larsbutler/mongoengine,eli-b/mongoengine