Warning in docs about frozen classes no longer applicable #816

In [1]: import attr

In [2]: @attr.frozen
   ...: class SlottedFrozen:
   ...:     foo: int
   ...:     bar: int
   ...:     baz: str
   ...:     bing: float = 5.0
   ...: 

In [3]: @attr.define
   ...: class SlottedUnfrozen:
   ...:     foo: int
   ...:     bar: int
   ...:     baz: str
   ...:     bing: float = 5.0
   ...: 

In [4]: @attr.s(slots=True, frozen=True, auto_attribs=True)
   ...: class OldSlottedFrozen:
   ...:     foo: int
   ...:     bar: int
   ...:     baz: str
   ...:     bing: float = 5.0
   ...: 

In [5]: @attr.s(slots=True, auto_attribs=True)
   ...: class OldSlottedUnfrozen:
   ...:     foo: int
   ...:     bar: int
   ...:     baz: str
   ...:     bing: float = 5.0
   ...: 

In [6]: %timeit SlottedFrozen(7, 42, "hi!")
553 ns ± 10.6 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [7]: %timeit SlottedUnfrozen(7, 42, "hi!")
566 ns ± 17.2 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [8]: %timeit OldSlottedFrozen(7, 42, "hi!")
553 ns ± 8.64 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [9]: %timeit OldSlottedUnfrozen(7, 42, "hi!")
296 ns ± 3.09 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [14]: print(inspect.getsource(OldSlottedUnfrozen.__init__))
def __init__(self, foo, bar, baz, bing=attr_dict['bing'].default):
    self.foo = foo
    self.bar = bar
    self.baz = baz
    self.bing = bing

In [15]: print(inspect.getsource(SlottedUnfrozen.__init__))
def __init__(self, foo, bar, baz, bing=attr_dict['bing'].default):
    _setattr = _cached_setattr.__get__(self, self.__class__)
    _setattr('foo', foo)
    _setattr('bar', bar)
    _setattr('baz', baz)
    _setattr('bing', bing)