More verbose warning.

Author: rikba

polygon_coverage_ros/include/polygon_coverage_ros/coverage_planner.h

@@ -218,8 +218,10 @@ class CoveragePlanner : public PolygonPlannerBase {
           ROS_WARN("No altitude specified. Creating default altitude of 1m.");
           altitude_ = std::make_optional(1.0);
         } else if (altitude_.value() < std::numeric_limits<double>::epsilon()) {
-          ROS_WARN("Altitude to small %.3fm. Setting default altitude of 1m.",
-                   altitude_.value());
+          ROS_WARN(
+              "Altitude to small %.3fm. Please set altitude through first "
+              "z-value of polygon. Setting default altitude of 1m.",
+              altitude_.value());
           altitude_ = std::make_optional(1.0);
         }
         sensor_model_ = std::make_shared<Frustum>(