clarify base64url definition

[Sakurann]

resolves #236

[c2bo]

I like the change - makes it pretty clear imho

2 parts that I think should also be slightly adjusted / wasn't sure about that also reference base64url encoding.

761:

The following is a non-normative example of the payload of the JWT used in the example above before base64url encoding and signing:

--> something like

The following is a non-normative example of the payload of the JWT used in the example above before getting base64url-encoded and signed:

1073 - not sure about this one:

(1) The Verifier produces a nonce value by generating at least 16 fresh, cryptographically random bytes with sufficient entropy, associates it with the session and base64url encodes it.