As to whether it's written down, that's debateable. National Rail Enquiries have recently added pages to their website with the validity codes and their explanations for each journey you put into the journey planner (with some exceptions). Finding this information isn't easy though - it's slightly buried and is a tortuous process to find.
First put in your journey criteria. Then when you get to the list of trains click 'check fares'. Select the service(s) you intend to use then scroll down to the blue summary box marked 'Tickets'. The name of your ticket type will be underlined, i.e.
Off-Peak Day Return, click on this and you will be directed to a page titled '
Ticket Terms and Conditions'. Scroll down to '
Restrictions' and buried in the text is the line:
view the specific 'Validity Code' applicable to the journey plan that you have selected. Click on this and you are finally(!) presented with the restriction information applicable to your ticket!
For those of you out there interested in validity codes, the full list (well nearly!) relating to off-peak and super off-peak journeys can be found here:
http://www.nationalrail.co.uk/pdfs/OffPeak.pdf