documentation plan?

["Jessica N. Lange" <jlange -at- oee -dot- com>]

I have sometimes seen references by posters here
to something called a "documentation plan".

Can someone explain to me what this is, 
and/or point me to books or web sites that 
explain it?

(I suspect this is something I already do without
calling it such, but one never knows, does one?)

thanks
Jessica