In one of the agile discussion forums following question was asked and the answer made sense to me(in bold)
>I have tried to limit documentation and verbally brief my >engineers on what I want doing - normally a white board >discussion. Ask them to work in pairs, keep documentation to a> minimum and get every one to understand the design by >verbal communication.
Limiting documentation isn't the world's greatest idea. It's better to let them figure out what they actually need and what's a waste of time."