tests/qapi-schema: Document alternate's enum lacks visit function

We generate a declaration, but no definition.

The QMP schema has two: Qcow2OverlapChecks and BlockdevRef.  Neither
visit_type_Qcow2OverlapChecksKind() nor visit_type_BlockdevRefKind()
is actually used.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

tests/qapi-schema/qapi-schema-test.json

@@ -56,6 +56,7 @@
 
 { 'alternate': 'UserDefAlternate',
   'data': { 'uda': 'UserDefA', 's': 'str', 'i': 'int' } }
+# FIXME only a declaration of visit_type_UserDefAlternateKind() generated
 
 { 'struct': 'UserDefC',
   'data': { 'string1': 'str', 'string2': 'str' } }