document what the method does

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

lib/private/Authentication/Token/IProvider.php

@@ -53,6 +53,8 @@ interface IProvider {
 	public function getToken($tokenId) ;
 
 	/**
+	 * Duplicate an existing session token
+	 *
 	 * @param string $oldSessionId
 	 * @param string $sessionId
 	 * @throws InvalidTokenException