vlib: add note for Plus_head struct

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@56237 15284696-431f-4ddb-bdfa-cd5b030d7da7

include/vect/dig_structs.h

@@ -746,6 +746,9 @@ struct Cat_index
   \brief Basic topology-related info
 
   Holds basic topology-related information about vector map
+
+  Important note: you should NOT store non-topological information in
+  topological structures.
 */
 struct Plus_head
 {