Rollup merge of #73762 - poliorcetics:trait-keyword, r=KodrAus
Rollup merge of #73762 - poliorcetics:trait-keyword, r=KodrAus

Document the trait keyword

Partial fix of #34601.

This document the trait keyword. To avoid doing too much and forcing more updates as functionalities evolve, I put two links to the reference, especially for trait objects. This mainly documents the "big" parts, not so much the small details that might trip someone experimenting.

@rustbot modify labels: T-doc,C-enhancement

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust