Document addReducer's new option 'streamable'
Users can now ask that their custom reductions be processed in a streaming fashion when registering their reduction function via addReducer(). This feature is mentioned in the upcoming release notes but is not in the manual.
Also slightly related, section 16.1 says 'If different contribute calls pass different callbacks, some (unspecified, unreliable) callback will be chosen for use.' That is actually an error that we abort on, so that text should be updated.