* Add an HTTP header that will influence on the cache
*
* @param string $header Header name
- * @param array|null $option
- * @todo FIXME: Document the $option parameter; it appears to be for
- * X-Vary-Options but what format is acceptable?
+ * @param string[]|null $option Options for X-Vary-Options. Possible options are:
+ * - "string-contains=$XXX" varies on whether the header value as a string
+ * contains $XXX as a substring.
+ * - "list-contains=$XXX" varies on whether the header value as a
+ * comma-separated list contains $XXX as one of the list items.
*/
public function addVaryHeader( $header, $option = null ) {
if ( !array_key_exists( $header, $this->mVaryHeader ) ) {