Document how oidc merges data from form and OP

[aeneasr]

Is your feature request related to a problem? Please describe.

We need to better document how the oidc strategy merges data.

Describe the solution you'd like

Looking at the code, the process seems to be as follows:

1. Hitting "sign up with ..." makes a POST request to /auth/browser/methods/oidc/auth/:request

• After completing OIDC dance, we hit /auth/browser/methods/oidc/callback/:provider
• If the identity schema can not be validated (e.g. the OP didn't return an email address), we return back to the registration UI with some error context (e.g. email is missing). Here, we expect to have all the fields with their values (e.g. username) set in oidc.ReuqestMethod.Config.Fields. This data is the result of applying the oidc schema and following the "mappings".
• The user fills out some of the data and makes another POST request to /auth/browser/methods/oidc/auth/:request, this time with a form field like email: foo@bar.