2.x: buffer documentation incorrect for onError behaviour

[adamsp]

The documentation for most buffer operators reads as follows:

When the source ObservableSource completes or encounters an error, the resulting ObservableSource emits the current buffer and propagates the notification from the source ObservableSource.

This is incorrect; none of the buffer operators emit their buffer before propagating an error:

@Override
public void onError(Throwable t) {
    buffer = null;
    actual.onError(t);
}

Note that this documentation was fixed in 1.x in #3561 (this is effectively a duplicate of issue #3560).