Document how to rename associations to avoid error 19

[TimLethbridge]

The code below gives error 19. However it is a bit obscure how to fix it and users have been confused. This example and the solution need to be added to file build/reference/9019NoDuplicateAssociations.txt
so it appears in the user manual

The problem generating error 19

class Element {}

associationClass Assignment {
  Date addedToGrouping;
  * Element group;
  * Element children;
}

The solution would be:

class Element {}

associationClass Assignment {
  Date addedToGrouping;
  * ch Element group;
  * gr Element children;
}