where is the document for namespace of move_group_interface

edit

In book, Master ROS for Robotics Programming. I notice this header file below

//MoveIt! header file
#include <moveit/move_group_interface/move_group.h>
int main(int argc, char **argv)
{
ros::init(argc, argv, "test_random_node",
ros::init_options::AnonymousName);
// start a ROS spinning thread
ros::AsyncSpinner spinner(1);
spinner.start();
// this connects to a running instance of the move_group node
// Here the Planning group is "arm"
move_group_interface::MoveGroup group("arm");
// specify that our target will be a random one
group.setRandomTarget();
// plan the motion and then move the group to the sampled target
    group.move();
    ros::waitForShutdown();
}

I tried to find the document for this header, but just found planning_interface namespace for move_group.h file , instead of move_group_interface namespace.

QUESTION1:

Why?

QUESTION2:

Or, maybe actually, it is not a question for ROS but for C++ instead?

EDIT1

Document of MoveGroupInterface, where it shows the header:

#include <move_group_interface.h>

QUESTION3:

If I want to use MoveGroupInterface class, how should I write the header correctly in my own code?