list: update comment to state that the zero value is ready to use.

R=gri
CC=golang-dev
https://golang.org/cl/2147042

src/pkg/container/list/list.go

@@ -25,6 +25,7 @@ func (e *Element) Next() *Element { return e.next }
 func (e *Element) Prev() *Element { return e.prev }
 
 // List represents a doubly linked list.
+// The zero value for List is an empty list ready to use.
 type List struct {
 	front, back *Element
 	len         int