Fix documentation of `RCLCPP_[INFO,WARN,...]` (#1943)
Fix documentation of `RCLCPP_[INFO,WARN,...]` (#1943)

In the pull request https://github.com/ros2/rclcpp/pull/1442 the ability
to use `std::string` as the first argument to the logging functions was
(rightfully) removed.

This commit just corrects the documentation of the macro, since it
confused me a bit ;)

Signed-off-by: Daniel Reuter <9126ce7210890a9d575946ba5fa55c83c76d4730@googlemail.com>
ros2/rclcpp,ros2/rclcpp