helper/schema: update docs

helper/schema/schema.go

@@ -31,6 +31,9 @@ type Schema struct {
 	// If one of these is set, then this item can come from the configuration.
 	// Both cannot be set. If Optional is set, the value is optional. If
 	// Required is set, the value is required.
+	//
+	// One of these must be set if the value is not computed. That is:
+	// value either comes from the config, is computed, or is both.
 	Optional bool
 	Required bool