OIDC: update documentation for wlcg profile

[paulmillar]

As pointed out in #7575, The Book's documentation on gPlazma / OIDC / WLCG profile is currently missing some important information:

• what is an explicit authorisation statement (the presence of which results in a token being considered an authorisation token),
• what is the consequences of the token being considered an authorisation token (or a non-authorisation token).