How does one find information about ROS Packages and find out how to use them.

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Hello ROS-folks,

I am trying to modify a code. In this code, it makes the user able to put two pointers in a client (ie. Rviz) and then the robot will travel between these two points. When it has reached that point it will switch it and then go back to its start. What I am trying to do is to create more points and then make it follow those pointers. The program uses the following packages:

#include <algorithm>
#include <ros/ros.h>
#include <actionlib/client/simple_action_client.h>
#include <actionlib/client/simple_client_goal_state.h>
#include <geometry_msgs/PointStamped.h>
#include <move_base_msgs/MoveBaseAction.h>
#include <visualization_msgs/Marker.h>
#include <visualization_msgs/MarkerArray.h>

After reading the code I am very certain that the package I need to change is <geometry_msgs/PointStamped.h> and I am also in my research i became very certain that the package I need to change it with is <geometry_msgs/polygonstamped.msg> (my logic: because i need more points and when points are added together it becomes a polygon). My question is how do i find infomation about the PolygonStamped on how to use it. The RAW definitions i find doesnt help on how to use it.

I am new to ROS and C++ so I hope the large description of my problem helps you guys to see my shortcomings of my expertise on this subject and help in the correct direction of what to learn so I can learn this myself.