Documenting a turn-around arc doesn't seem to me to be necessary. As long as the TA point is documented, the course layout team can set up a semicircular TA as in the image Jim posted above. As long as everyone runs around this point, they aren't shortening, they are slightly lengthening the course, "of course". Some runners naturally take this kind of an arc at turn arounds - particularly faster runners.
In the 1983 Cherry Blossom 10 Miler, Greg Myer tried to turn too sharply at a single point TA. He fell and opened a gash on his leg. It was astonishing to see him run past everyone with blood running down into his shoe. He still set the American record for 10 miles that day. 8 days later he won the Boston Marathon.