added documentation for preFilter
added documentation for preFilter

melalj/maptable,Packet-Clearing-House/maptable,Packet-Clearing-House/maptable,Packet-Clearing-House/maptable,Packet-Clearing-House/maptable,melalj/maptable,melalj/maptable,melalj/maptable,melalj/maptable