Fixed incorrect comment for ParentWindow (#615, #604)

imgui_internal.h

@@ -367,7 +367,7 @@ struct ImGuiContext
     ImGuiID                 HoveredIdPreviousFrame;
     ImGuiID                 ActiveId;                           // Active widget
     ImGuiID                 ActiveIdPreviousFrame;
-    bool                    ActiveIdIsAlive;
+    bool                    ActiveIdIsAlive;                    // Active widget has been seen this frame
     bool                    ActiveIdIsJustActivated;            // Set at the time of activation for one frame
     bool                    ActiveIdAllowOverlap;               // Set only by active widget
     ImVec2                  ActiveIdClickOffset;                // Clicked offset from upper-left corner, if applicable (currently only set by ButtonBehavior)
@@ -641,7 +641,7 @@ struct IMGUI_API ImGuiWindow
     ImDrawList*             DrawList;
     ImGuiWindow*            RootWindow;                         // If we are a child window, this is pointing to the first non-child parent window. Else point to ourself.
     ImGuiWindow*            RootNonPopupWindow;                 // If we are a child window, this is pointing to the first non-child non-popup parent window. Else point to ourself.
-    ImGuiWindow*            ParentWindow;                       // If we are a child window, this is pointing to our parent window. Else point to NULL.
+    ImGuiWindow*            ParentWindow;                       // Immediate parent in the window stack *regardless* of whether this window is a child window or not)
 
     // Navigation / Focus
     int                     FocusIdxAllCounter;                 // Start at -1 and increase as assigned via FocusItemRegister()