Rollup merge of #139654 - nnethercote:AssocKind-descr, r=compiler-errors

Improve `AssocItem::descr`.

The commit adds "associated" to the description of associated types and associated consts, to match the description of associated functions. This increases error message precision and consistency with `AssocKind::fmt`.

The commit also notes an imperfection in `AssocKind::fmt`; fixing this imperfection is possible but beyond the scope of this PR.

r? `@estebank`

Author: Zalathar