Document `const_fn_transmute` and `const_fn_union`

The changes for https://github.com/rust-lang/rust/pull/85769 need to documented (probably around [here](https://github.com/rust-lang/reference/blob/master/src/const_eval.md)). See https://github.com/rust-lang/rust/pull/85769#issuecomment-854363720 for the stabilization report.