Clarify comment about ID_RECALC_COPY_ON_WRITE

The copy-on-write is really an implementation detail of the
dependency graph. While there are still cases where there is
no better tag to be used, the ID_RECALC_COPY_ON_WRITE should
not be used in combination with a dedicated tag.

For example if location of object changes the proper tag is
`ID_RECALC_TRANSFORM`. Tagging with `ID_RECALC_TRANSFORM |
ID_RECALC_COPY_ON_WRITE` will seemingly work, but this is
not an intended usage.

source/blender/makesdna/DNA_ID.h

@@ -914,8 +914,9 @@ typedef enum IDRecalcFlag {
   ID_RECALC_EDITORS = (1 << 12),
 
   /* ** Update copy on write component. **
-   * This is most generic tag which should only be used when nothing else
-   * matches.
+   *
+   * This is most generic tag which should only be used when nothing else matches.
+   * It is not to explicitly mixed in with other recalculation flags.
    */
   ID_RECALC_COPY_ON_WRITE = (1 << 13),