I tried keeping such documentation. But it isn't that helpful, and it goes out of date too quickly.
Code requires that all electrical connections be made inside junction boxes, and all junction boxes remain accessible forever.
That means that your entire house's electrical system contains only 2 things: a) junction boxes which are accessible, and b) cables or conduits connecting junction boxes which have no joins or splices in them.
This creates a world where wiring is largely "self-documenting" - one needs to simply look at where the wiring is likely to go, and open up junction boxes in that vicinity. This reveals everything that there is to see. Assuming rules were not violated. Circuits can be ascertained by shutting off a breaker and seeing what goes dead, and there are more sophisticated pro tools to trace any given cable leaving a box, but they're usually not necessary. An experienced person can usually just tell by recognizing the connection patterns given the usage.
Also, the most important documentation method is how the wiring is hooked up presently. About twice a year, a novice opens up a complicated box, unhooks every single wire from whichever terminal or wire nut it is on, splays out all the individual wires and takes a photo and posts it here. "How do I hook this up?" they ask. Well, we can't tell them because they just destroyed all the clues! So yeah - take great heed for how it is connected.
