update docs around use_union

[oconnor663]

The docs still say that use_union requires the nightly channel, but it looks like it no longer does? (Though it also sounds like there are some safety questions still open?)

[bluss]

Yes, the mishap with going over to ManuallyDrop there was a bit unfortunate. It's a good idea to put it on a bit safer ground (aligning a bit with the experimental PR). I don't want to update the docs; it's an experimental / unstable feature in arrayvec.

Good course of action would be to nullify the feature (make it a no op) and introduce a new one.

[bluss]

Closed by #114