improve docstrings a bit for new struct field
diff --git a/parser.go b/parser.go
index bfeb00c..4bc9441 100644
--- a/parser.go
+++ b/parser.go
@@ -24,6 +24,11 @@
 	// NamespaceDelimiter separates group namespaces and option long names
 	NamespaceDelimiter string
+	//UnknownOptionsHandler is a function which, if non-nil, is called
+	//when the parser encounters an option that is unmapped.
+	//It takes the option name and remaining args to be parsed, and should
+	//return the (modified, if necessary) args, and a non-nil error if the
+	//handler fails.
 	UnknownOptionHandler func(option string, args []string) ([]string, error)
 	internalError error