Confusion about whether union writes ever have to be placed in `unsafe` blocks

https://doc.rust-lang.org/reference/items/unions.html says:

Writes to Copy or ManuallyDrop union fields do not require reads for running destructors, so these writes don't have to be placed in unsafe blocks

but later also says:

When a union is dropped, it cannot know which of its fields needs to be dropped. For this reason, all union fields must either be of a Copy type or of the shape ManuallyDrop<_>.

If the latter is true, then the former seems to be written in an overly-qualified way.

Start with the union section in the Rust Reference at items/unions.html and compare the two quoted statements about union field writes and dropping. Clarify the wording so the rules no longer appear contradictory; the issue is resolved when the section accurately explains when union writes require unsafe blocks.