From where I stand, this is just one more example of members of the Rust community working hard to find excuses for not having documentation.

To the extent that people are relying on the common cases of the niche optimisation taking place: yes, the Rust description should be documenting what people may rely on, and should have been documenting it for years.

In order to do that there's absolutely no need to describe the fine details of exactly where the optimisation currently applies.