ImVector comment (#1577)

imgui.h

@@ -1073,6 +1073,7 @@ public:
         Capacity = new_capacity;
     }
 
+    // NB: &v cannot be pointing inside the ImVector Data itself! e.g. v.push_back(v[10]) is forbidden.
     inline void                 push_back(const value_type& v)  { if (Size == Capacity) reserve(_grow_capacity(Size + 1)); Data[Size++] = v; }
     inline void                 pop_back()                      { IM_ASSERT(Size > 0); Size--; }
     inline void                 push_front(const value_type& v) { if (Size == 0) push_back(v); else insert(Data, v); }