[Documentation] want_authn_requests_only_with_valid_cert and friends

[peppelinux]

Hello everybody!

I think that these few notes could be helpful to a future PR to pysaml Documentation:

want_authn_requests_only_with_valid_cert
This option make mandatory the presence of the SP cert in a (signed) AuthnRequest. The presence of a statement like the following:

        <ds:KeyInfo>
            <ds:X509Data>
                <ds:X509Certificate>MIIDFzCCA [ ... that thing ...] wy32Ee</ds:X509Certificate>
            </ds:X509Data>
        </ds:KeyInfo>

And then we should cover also the meaning and the usage of the following

            'validate_certificate': True,
            # this is default
            'only_use_keys_in_metadata': True,

[peppelinux]

Done here:
#715