Fix visibility and add documentation (#1407)

Signed-off-by: Michael Carroll <[email protected]>

src/EntityComponentManagerDiff.hh

@@ -31,18 +31,37 @@ namespace ignition
     // Inline bracket to help doxygen filtering.
     inline namespace IGNITION_GAZEBO_VERSION_NAMESPACE {
 
-    class EntityComponentManagerDiff
+    /// \\brief Used to track the changes in an EntityComponentManager
+    ///
+    /// Tracks added and removed entities for the purpose of a reset
+    class IGNITION_GAZEBO_VISIBLE EntityComponentManagerDiff
     {
+      /// \brief Add an added entity to the current diff
+      /// \param[in] _entity Entity that was added
       public: void InsertAddedEntity(const Entity &_entity);
+
+      /// \brief Add a removed entity to the current diff
+      /// \param[in] _entity Entity that was removed
       public: void InsertRemovedEntity(const Entity &_entity);
 
-      public: const std::vector<Entity> &RemovedEntities() const;
+      /// \brief Retrieve the list of added entities
+      /// \return Vector of Entity IDs added since construction/clear
       public: const std::vector<Entity> &AddedEntities() const;
 
+      /// \brief Retrieve the list of removed entities
+      /// \return Vector of Entity IDs removed since construction/clear
+      public: const std::vector<Entity> &RemovedEntities() const;
+
+      /// \brief Clear the list of added entities
       public: void ClearAddedEntities();
+
+      /// \brief Clear the list of removed entities
       public: void ClearRemovedEntities();
 
+      /// \brief List of added entities
       private: std::vector<Entity> addedEntities;
+
+      /// \brief List of removed entities
       private: std::vector<Entity> removedEntities;
     };
   }