Docs: Note storage behavior of override operations in comment

It's not obvious that override property operations are recreated on undo
pushes, which caused me some confusion. Comment on this, so it's clear
that pointers to the operations shouldn't be stored.

source/blender/makesdna/DNA_ID.h

@@ -256,6 +256,7 @@ typedef struct IDOverrideLibraryProperty {
 
   /**
    * List of overriding operations (IDOverrideLibraryPropertyOperation) applied to this property.
+   * Recreated as part of the diffing, so do not store any of these elsewhere.
    */
   ListBase operations;