Rollup merge of #73867 - poliorcetics:union-keyword, r=joshtriplett
Rollup merge of #73867 - poliorcetics:union-keyword, r=joshtriplett

Document the union keyword

Partial fix of #34601.

This documents the `union` keyword by presenting three cases: simply using a union, matching on a union and referencing the fields of a union.

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

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