Fix the documentation of the atomic_hook.h registration functions to correctly state that only the first registered hook will be honored.

The comments that imply otherwise were never true, and were a leftover artifact during initial development of the feature.

Also remove a TODO() I gave myself years ago; this is never going to happen and isn't worth the bother.

PiperOrigin-RevId: 495687371
Change-Id: I63f8ef57d659075bf290caae0617ea61ceb2c1db