The audience would be the very few people who actually care about what's going on inside the guts of malloc().

So mostly people writing high performance code and wondering things like how does it perform in threaded code, whether there are optimizations for small allocations, etc. And also very valuable to the very, very few people who might think "I can do better" and contribute an improvement.

Documentation about the deep internals of stuff can be extremely valuable, but since very few people need it, it tends to be a very rare find.