Update filtering documentation - fixes #306
Update filtering documentation - fixes #306

Explains that filtering is done relative to the remote root.

Also removes a section that seems more about internal knowledge and
that may likely more confuse people. Adds instead a section giving an
overview of how to perform filtering before going into details.

ncw/rclone,olihey/rclone,jackschmidt/rclone,olihey/rclone,jackschmidt/rclone,olihey/rclone,yonjah/rclone,olihey/rclone,seuffert/rclone,kynikos/rclone,B4dM4n/rclone,B4dM4n/rclone,kynikos/rclone,seuffert/rclone,arcimboldo/rclone,yonjah/rclone,arcimboldo/rclone,B4dM4n/rclone,ncw/rclone,seuffert/rclone,ncw/rclone,jackschmidt/rclone,arcimboldo/rclone,seuffert/rclone,yonjah/rclone,kynikos/rclone,arcimboldo/rclone,ncw/rclone,kynikos/rclone,B4dM4n/rclone,yonjah/rclone,jackschmidt/rclone