[docs] Don't display has_many as an attribute.

[przprz]

The docs reads

//! You can declare an association between two records with `#[belongs_to]`.
//! Unlike other ORMs, Diesel has no concept of `#[has_many`]

If there's no has many in Diesel, why use the argument syntax here?
I found it a little bit confusing when reading for the first time.

[weiznich]

Filled rust-lang/rust#79560 for the nightly failure (it's a upstream rustc bug)