Document that typing_extensions.Protocol and typing.Protocol don't always play well together

[AlexWaygood]

Closes #245

[AlexWaygood]

@JelleZijlstra would it be possible to set up readthedocs previews for docs PRs? :)

[JelleZijlstra]

@JelleZijlstra would it be possible to set up readthedocs previews for docs PRs? :)

I've procrastinated figuring out how to do that :)

[JelleZijlstra]

Building locally gives

/Users/jelle/py/typing_extensions/doc/index.rst:307: WARNING: term not in glossary: 'method-resolution-order'

[AlexWaygood]

Building locally gives

/Users/jelle/py/typing_extensions/doc/index.rst:307: WARNING: term not in glossary: 'method-resolution-order'

I'm on my phone, but I think my last push should fix that!

[JelleZijlstra]

Looks great!