Describe the meaning of 0 for Touch()'s seconds parameter (bradfitz#82)

This is mentioned in the Expiration field of the Item struct (no
expiration time), but not in the notes about the Touch function.

memcache/memcache.go

@@ -325,8 +325,9 @@ func (c *Client) Get(key string) (item *Item, err error) {
 
 // Touch updates the expiry for the given key. The seconds parameter is either
 // a Unix timestamp or, if seconds is less than 1 month, the number of seconds
-// into the future at which time the item will expire. ErrCacheMiss is returned if the
-// key is not in the cache. The key must be at most 250 bytes in length.
+// into the future at which time the item will expire. Zero means the item has
+// no expiration time. ErrCacheMiss is returned if the key is not in the cache.
+// The key must be at most 250 bytes in length.
 func (c *Client) Touch(key string, seconds int32) (err error) {
 	return c.withKeyAddr(key, func(addr net.Addr) error {
 		return c.touchFromAddr(addr, []string{key}, seconds)