Make output_stream blurb Doxygen
Make output_stream blurb Doxygen

This comprehensive description of a much-used public class should be
visible in external documentation, like others.

Signed-off-by: Dejan Mircevski <1e9694aae17a51d4875d9bf4a0d0e20ef37c1b85@scylladb.com>
Message-Id: <20190911133931.53716-1-1e9694aae17a51d4875d9bf4a0d0e20ef37c1b85@scylladb.com>

scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar,dreamsxin/seastar,dreamsxin/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,cloudius-systems/seastar,dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar,scylladb/seastar