Document "the Any trick"

[Avasam]

Addresses the documentation part of #11094 .

Copied almost directly from #7770 (comment) just to get the ball rolling.
Wordsmith away! I don't feel strongly about any particular description, as long as it ends up documented and explained!

I'm not including an alias here because getting "the Any trick" documented at all is the most important part we all agreed on. I also noticed that it's not always clear why an Any union is used. So an alias for those cases should definitely help, but will require some thinking over changes. When creating the alias, it can immediately be used in stdlib as well. And a small mention can be added to the this here documentation. Something like:

### "The `Any` trick" and `MaybeNone`
...

To help with semantics and avoid confusion, we defined an alias `MaybeNone` to be used in the same way. Indicating that the developper can be ready to deal with a potentially `None` value, and let type-checkers know that type-guarding on `None` is not redundant.

[AlexWaygood]

+1 for adding docs first — it doesn't preclude adding an alias at a later date, and it's easier to get consensus on!

[Akuli]

A pile of small nits that should make this easier to understand.

[Akuli]

This is pretty good. A few more nits.

[Akuli]

Looks good to me, but another review would be good IMO.