Document the order of values returned by keyAt().

The values returned by keyAt() are currently guaranteed to be in
ascending order. This is helpful to users of the API to be able to make
assumptions about the keys and values when iterating over one of the
sparse array implementations.

This commit adds documentation about this.

Change-Id: I3d7eb78e115ce174f1167b83904b44bf5120b223
6 files changed