Update rfc-structural-generic-instantiation.md

Author: raph-amiard

considered/rfc-structural-generic-instantiation.md

@@ -205,8 +205,11 @@ end F;
 * Generics annotated with the `Allow_Structural_Instantiation` aspect are
   forbidden to have:
 
-  - Mutable global state
-  - In out object formals
+  - Mutable global state - TODO refine
+  - Non in object formals
+
+* Additionally, instantiations of those generics can only pass static expressions
+  for object formals.
 
 * Generics annotated with the `Allow_Structural_Instantiation` are forbidden to
   have no generic formals.