Improved doc for SequenceField
Improved doc for SequenceField

Related to issue #497
jimmyshen/mongoengine,Davidrjx/mongoengine,khallaghi/mongoengine,DavidBord/mongoengine,nicolascine/mongoengine,muhammad-ammar/mongoengine,Catstyle/mongoengine,MakerReduxCorp/mongoengine,prijindal/mongoengine,iandd0824/mongoengine,losintikfos/mongoengine,gmacon/mongoengine,9nix00/mongoengine,yarneo/mongoengine,Davidrjx/mongoengine,gavincabbage/mongoengine,ProgressivePlanning/mongoengine,sourcelair/mongoengine,nathanial/mongoengine,MongoEngine/mongoengine,MRigal/mongoengine,touilleMan/mongoengine,MakerReduxCorp/mongoengine,WingGao/mongoengine,larsbutler/mongoengine,blaggacao/mongoengine,RussellLuo/mongoengine,cogniteev/mongoengine,reallistic/mongoengine,ConsumerPhysics/mongoengine,cogniteev/mongoengine,eli-b/mongoengine