Call out non-guaranteed order of $unionWith

Description

Currently, $unionWith outputs documents in a predictable order. But it's not guaranteed: users should assume that $unionWith can output documents in any order.

The page for $unionWith doesn't mention ordering one way or the other. It would be good to explicitly warn people that the order is not guaranteed. (We've done this for other operators, like $addToSet.)

The Versioned API Scope talks about why we don't want to promise the output order will stay predictable: it leaves the door open for us to optimize it better in the future.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)