Add linearity notes to Enum.fetch and friends
Improve description of operations that takes linear time
Update doc for GenEvent.stream/2

- GenEvent streams are not two way, so remove language suggesting that
you can notify via the streams.
- Clarify that consumption doesn't start until we enumerate.
Merge pull request #2539 from hamiltop/update_genevent_stream_doc

Update doc for GenEvent.stream/2