Update ROS API documentation. (#450)
Update ROS API documentation. (#450)

Add mentions of the offline node and the occupancy grid node.

Fixes #448.
cartographer-project/cartographer_ros,googlecartographer/cartographer_ros,cartographer-project/cartographer_ros,wohe/cartographer_ros,wohe/cartographer_ros,damonkohler/cartographer_ros,googlecartographer/cartographer_ros,memorysafe/cartographer_ros,cartographer-project/cartographer_ros,googlecartographer/cartographer_ros,wohe/cartographer_ros,damonkohler/cartographer_ros,memorysafe/cartographer_ros,damonkohler/cartographer_ros,memorysafe/cartographer_ros