[stdlib] Add note to swap() docstring about non-aliasing requirement

[lilyball]

No description provided.

[gribozavr]

This is not specific to swap, this is a general requirement for inout. It might be worth calling out here, true, but we need to be conscious that we won't catch all places with notes like this.

[lilyball]

True, this just seemed like the spot with the highest risk of aliasing. For example, when shuffling a list, it's easy to forget to check that you aren't trying to swap a location with itself, so calling it out specifically in the documentation is a good reminder.

[NachoSoto]

In fact this bit me in the Swift 1.2 -> 2 transition IIRC. A naively-implemented shuffle method started crashing because of this change in behavior, so I agree it would be worth calling out.