Rollup merge of #117070 - notriddle:notriddle/cleanx, r=fmease

rustdoc: wrap Type with Box instead of Generics

When these `Box<Generics>` types were introduced, `Generics` was made with `Vec` and much larger. Now that it's made with `ThinVec`, `Type` is bigger and should be boxed instead.

Author: matthiaskrgr