Where is this documented? I've not heard of it before.
It isn't, it's just how it conventionally worked in BR days with things like Savers, and is largely my experience[0]. But BR was never quite so prosecution-happy as the present-day operators, nor were there quite so many fares anomalies that might make you want to do it, nor were most stations barriered in the days when this sort of thing came up in that way.
However, with a walk-up ticket you near enough always have a get-out that will prevent prosecution unless you are on the last train or similar. If stopped and told you can't break your journey, simply return to the platform and continue it to the ticketed destination on the next valid train. This might be inconvenient, but as Break of Journey has not occurred until you have left railway premises, there would only be a chance of issues (legally[1]) were an inspector waiting at the building's exit, which while it does occasionally happen I have never witnessed myself.
With an Advance, of course, you do not have that option, as the second the train leaves without you on it you are stuck. And starting short is of course a whole different ball game.
[0] I have often broken journeys where BoJ was not technically permitted, e.g. to go to the shops between trains, and I have never, ever been even slightly questioned about it.
[1] I'm not saying you might not end up with something against you, but you would win in court because Break of Journey is very clearly defined as leaving Railway property with certain specific exceptions.
