added note to QueryInterface::where()

framework/db/QueryInterface.php

@@ -153,6 +153,8 @@ interface QueryInterface
      * - Additionally you can specify arbitrary operators as follows: A condition of `['>=', 'id', 10]` will result in the
      *   following SQL expression: `id >= 10`.
      *
+     * **Note that this method will override any existing WHERE condition. You might want to use [[andWhere()]] or [[orWhere()]] instead.**
+     *
      * @param array $condition the conditions that should be put in the WHERE part.
      * @return $this the query object itself
      * @see andWhere()