* Fix [Bug #19632]: Disable external iterator for frozen enumerator

Currently, methods to manipulate an external iterator like `#next`
and `#feed` can be called even if a receiver of an enumerator is
frozen. However, these methods change the state of an external
iterator in an enumerator. Therefore, it seems a BUG to me, and
these methods should raise FrozenError if the receiver is frozen.

This fixed the following methods to raise FrozenError if the receiver is
frozen.

- `Enumerator#next`
- `Enumerator#next_values`
- `Enumerator#peek`
- `Enumerator#peek_values`
- `Enumerator#feed`
- `Enumerator#rewind`

* Fix a typo in the document

Thanks @Maumagnaguagno.