Add notes to BaseStream.spliterator/iterator docs regarding them being escape hatches

The stream package doc states:

 * stream. In almost all cases, terminal operations are <em>eager</em>,
 * completing their traversal of the data source and processing of the pipeline
 * before returning. Only the terminal operations {@code iterator()} and
 * {@code spliterator()} are not; these are provided as an "escape hatch" to enable
 * arbitrary client-controlled pipeline traversals in the event that the
 * existing operations are not sufficient to the task.

Similar notes should be added to BaseStream.spliterator/iterator.