Navigator: Improve code readibility

This diff does not contain any functional changes but just makes the code more readable and adds comments.

src/modules/navigator/mission_block.h

@@ -70,21 +70,21 @@ public:
 	/**
 	 * Check if the mission item contains a navigation position
 	 *
-	 * @return false if mission item should not be part of the trajectory
+	 * @return false if the mission item does not contain a valid position
 	 */
 	static bool item_contains_position(const mission_item_s &item);
 
 	/**
 	 * Check if the mission item contains a gate condition
 	 *
-	 * @return true if mission item is neither a position nor a command
+	 * @return true if mission item is a gate
 	 */
 	static bool item_contains_gate(const mission_item_s &item);
 
 	/**
 	 * Check if the mission item contains a marker
 	 *
-	 * @return true if mission item is neither a position nor a command
+	 * @return true if mission item is a marker
 	 */
 	static bool item_contains_marker(const mission_item_s &item);