qapi: Make visit_next_list()'s comment less confusing

visit_next_list() returns non-null on success, null on failure.  The
comment's phrasing "until NULL return or error occurs" is needlessly
confusing.  Scratch the "or error occurs" part.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190802122325.16520-1-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

include/qapi/visitor.h

@@ -364,10 +364,10 @@ void visit_start_list(Visitor *v, const char *name, GenericList **list,
  * @tail must not be NULL; on the first call, @tail is the value of
  * *list after visit_start_list(), and on subsequent calls @tail must
  * be the previously returned value.  Should be called in a loop until
- * a NULL return or error occurs; for each non-NULL return, the caller
- * then calls the appropriate visit_type_*() for the element type of
- * the list, with that function's name parameter set to NULL and obj
- * set to the address of @tail->value.
+ * a NULL return; for each non-NULL return, the caller then calls the
+ * appropriate visit_type_*() for the element type of the list, with
+ * that function's name parameter set to NULL and obj set to the
+ * address of @tail->value.
  */
 GenericList *visit_next_list(Visitor *v, GenericList *tail, size_t size);