[docs] ssl.create_default_context

[Christoph Egger]

Hi!

```
The settings are: PROTOCOL_TLS, OP_NO_SSLv2, and OP_NO_SSLv3 with high encryption cipher suites without RC4 and without unauthenticated cipher suites. Passing SERVER_AUTH as purpose sets verify_mode to CERT_REQUIRED and either loads CA certificates (when at least one of cafile, capath or cadata is given) or uses SSLContext.load_default_certs() to load default CA certificates.
```

`SERVER_AUTH` not only sets `CERT_REQUIRED` but also `check_hostname` to `True`. As this is critical information for users with some familiarity with python’s SSL library the documentation should mention it (especially as it talks explicitly about the `CERT_REQUIRED` part)

https://github.com/python/cpython/blob/master/Lib/ssl.py#L567

Thanks!

  Christoph
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