clarify error message

Cython/Compiler/Nodes.py

@@ -4190,7 +4190,7 @@ class CClassDefNode(ClassDefNode):
                     else:
                         self.base_type = base_class_entry.type
                 if env.directives.get('freelist', 0) > 0:
-                    warning(self.pos, "freelists are not currently supported for subtypes", 1)
+                    warning(self.pos, "freelists cannot be used on subtypes, only the base class can manage them", 1)
 
         has_body = self.body is not None
         if self.module_name and self.visibility != 'extern':

tests/errors/e_exttype_freelist.pyx

@@ -20,5 +20,5 @@ cdef class ExtSubTypeFail(ExtType):
 
 
 _ERRORS = """
-18:5: freelists are not currently supported for subtypes
+18:5: freelists cannot be used on subtypes, only the base class can manage them
 """