tcp: clarify tcp_congestion_ops functions comments

tcp: clarify tcp_congestion_ops functions comments

The optional and required hints in the tcp_congestion_ops are information
for the user of this interface to signalize its importance when
implementing these functions.

However, cong_avoid comment incorrectly tells that it is required,
in reality congestion control must provide one of either cong_avoid or
cong_control.

In addition, min_tso_segs has not had any comment optional/required
hints. So mark it as optional since it is used only in BBR.

Co-developed-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Daniel Sedlak <daniel.sedlak@cdn77.com>
Link: https://patch.msgid.link/20260105115533.1151442-1-daniel.sedlak@cdn77.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>