Clarify the reference's status.

[steveklabnik]

The former wording only gave part of the picture, we want to be crystal
clear about this.

/cc @petrochenkov, who had concerns about #37820

[rust-highfive]

@steveklabnik: no appropriate reviewer found, use r? to override

[GuillaumeGomez]

Well, feel better about this but the previous solution was fine by me as well. :-/

[matklad]

👍 It is indeed very incomplete. I'd probably dropped necessarily word, because is incomplete is more direct and more true :)

Not sure about accuracy and freshness. I bet a thorough review would find at least a handful of false statements :)

It would be a great idea to add a big WIP warning to the reference itself, because the current introduction sounds too assuring: https://doc.rust-lang.org/reference.html#introduction

[GuillaumeGomez]

@bors: r+ rollup

[bors]

📌 Commit c5e6dfc has been approved by GuillaumeGomez