Changes

stream: osmo_stream_{cli,srv}_set_segmentation_cb(2): documentation improvements

* Wrap lines to a shorter width
* Improve osmo_stream_cli_set_segmentation_cb() adding text available in
  osmo_stream_srv_set_segmentation_cb()
* Refer to segmentation_cb in segmentation_cb2 documentation and only
  outline the differences.

Change-Id: I55eb9a7781c22db68446265d2a174ae4f5e9f6ef