Basic - 2 assertion definition

[Former user]

The term "assertion" is first used in section 3.3.4.2.

A definition appears later under security considerations, section 6.

The definition in section 6 is not very clear either: is the "result of the authentication returned to the client" the ID token, the response from the check session endpoint, either/both?

[Edmund Jay]

fixes #57

→ 58fa84bb0d2d