The null pointer optimization is documented in many places, here's one official instance I published in 2015: https://doc.rust-lang.org/nomicon/repr-rust.html
There are many random details of a language that are "important" and they get documented all over the place. If it was all under "THE REFERENCE" and was a 500 page PDF, would the information actually be more discoverable? How would you even know to look up what you don't know? Are you going to read the reference cover to cover before writing hello world?
If the people maintaining a language produce a complete and correct description, then thousands of other people around the world can use it to provide helpful and correct blog posts, Stack Overflow answers, and so on.
But those thousands of people, however enthusiastic, can't produce authoritative documentation, because that needs the maintainers' "sign-off".
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.