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?