`EndEntityCert` documentation refers to removed methods

[tannaurus]

EndEntityCert's documentation is refers to methods that don't exist when describing what needs to be validated for both client and server tls connections

The following functions were deprecated since 0.101.2 and subsequently removed:

• EndEntityCert.verify_is_valid_tls_server_cert
• EndEntityCert.verify_is_valid_tls_client_cert

There's also an additional reference to verify_is_valid_tls_server_cert in tests/generate.py

[djc]

Oops, that's probably my fault -- should have updated the docs in #133.

Please review the changes in #271.