My suggestion was not to delete the "approximate" entirely. Just move it out of the first sentence to make it more consistent with the other documentation.

This is the model I'm seeing in empty() and full():

The first sentence is something simple and direct (without nebulous words that will make people wonder what it means), and then after that, comment on the race-condition stuff.

I think that documentation is good, and it would be good to follow the same model for qsize().