This pull request makes the following changes to the table of methods in the [`collections.abc` documentation](https://docs.python.org/3/library/collections.abc.html):

- `Reversible`: add `__iter__` abstract method;
- `Generator`: replace `__iter__` with inherited mixin `Iterator` methods;
- `MutableSequence`: add clear mixin method;
- `Set`: remove `__ne__` mixin method (not defined here but in `object`), add `__rand__` mixin method, add `__ror__` mixin method, add `__rsub__` mixin method, add `__rxor__` mixin method;
- `Mapping`: remove `__ne__` mixin method (not defined here but in `object`);
- `ItemsView`: add inherited mixin `MappingView` method and inherited mixin `Set` methods;
- `KeysView`: add inherited mixin `MappingView` method and inherited mixin `Set` methods;
- `ValuesView`: add inherited mixin `MappingView` method;
- `Coroutine`: add `__await__` abstract method;
- `AsyncGenerator`: replace `__aiter__` with inherited mixin `AsyncIterator` methods;
- footnotes: remove footnote 2 which is a duplicate of [the description of `collections.abc.Iterable`](https://docs.python.org/3/library/collections.abc.html#collections.abc.Iterable).