rustdoc: Read and Write show up as notable traits on &mut [u16]

[jsha]

https://rustdoc.crud.net/jsha/fewer-colors/std/primitive.char.html#method.encode_utf16

[camelid]

@notriddle there are several issues which I think are duplicates of this and thus fixed by your change, but I'm not certain. Can you let me know if these can be closed?

• rustdoc: Notable traits shows wrong type parameter #80737
• rustdoc generates "Notable traits" that are impossible #76590
• Rustdoc suggests notable trait for &mut when given shared reference #78160