[Stdlib] Tweak doc comment for Indexable.startIndex

[lilyball]

The complexity of this property may actually be O(N) for some lazy
collections.

Fixes SR-425.

[dabrahams]

That isn’t how I want to handle SR-425. The conformance of LazyFilterCollection to CollectionType has always been, and will always be, a little fishy. We address that by aggressively commenting it as an imperfect model, rather than weakening other guarantees.

[lilyball]

@dabrahams How about FlattenCollection? It's not a LazyCollectionType, but it is still is inherently lazy in its own operation, so e.g. [[],[],[],[],[],[1]].flatten() is O(N) on startIndex (and on first).

[dabrahams]

How about it? If it doesn’t have appropriate caveats in the doc comments, it should.

[lilyball]

I've submitted #829 instead that updates doc comments on the lazy collections.