CircBuffer: docs: add that indices are no longer Strideable (#914)
CircBuffer: docs: add that indices are no longer Strideable (#914)

Motivation:

CircularBuffer indices are no longer `Strideable`, document this.

Modifications:

added to the public api changes doc

Result:

better docs
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio