Removed documentation of removed RiverNative unit class flag.
Removed documentation of removed RiverNative unit class flag.

tltneon/freeciv-web,adaxi/freeciv-web,adaxi/freeciv-web,adaxi/freeciv-web,andreasrosdal/freeciv-web,sylvain121/freeciv-web-server,tltneon/freeciv-web,tltneon/freeciv-web,tltneon/freeciv-web,tltneon/freeciv-web,andreasrosdal/freeciv-web,adaxi/freeciv-web,adaxi/freeciv-web,andreasrosdal/freeciv-web,andreasrosdal/freeciv-web,sylvain121/freeciv-web-server,tltneon/freeciv-web,andreasrosdal/freeciv-web,sylvain121/freeciv-web-server