Rollup merge of #48026 - Badel2:doc-assoc-const-object-safe, r=nikomatsakis
Rollup merge of #48026 - Badel2:doc-assoc-const-object-safe, r=nikomatsakis

Document that associated constants prevent a trait from being made into an object

Fixes https://github.com/rust-lang/rust/issues/47952

Add a short mention of associated constants to E0038

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