Remove outdated note about non-Copy union fields

[GoldsteinE]

Unions with non-Copy fields are now stable.

[Mark-Simulacrum]

I think this isn't quite right - only a small sunset of non-Copy types is permitted, and probably should be documented here.

[GoldsteinE]

I thought any non-Copy types without custom destructors are allowed? I’m going to re-read that issue, thanks for correction.

[ehuss]

We recently changed the documentation for union drop requirements (#1238). I think it would be fine to keep some kind of note here, just as a reminder as to why derive(Copy) is included in the example. Perhaps it could be something like:

Note: Union fields are restricted to a certain subset of types, such as deriving Copy in this example. See Unions for more information.

Also, there are two identical notes in this chapter that need to be updated.