Rollup merge of #42151 - Wallacoloo:docfix_into_vec, r=frewsxcv
Rollup merge of #42151 - Wallacoloo:docfix_into_vec, r=frewsxcv

Mention Vec::into_boxed_slice in documentation of [T]::into_vec

This is a documentation fix.

`Vec::into_boxed_slice` and `[T]::into_vec` are inverses, so it makes sense to mention the other in their respective documentation for visibility. `Vec::into_boxed_slice` already mentions `[T]::into_vec`, but not the other way around until now.

Tagging @steveklabnik per his request.

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