docs: small improvements to discord oidc provider guide

[NickUfer]

Proposed changes

• Adds reference to where the claims struct is built
• Removes statement that iss claim is added by email scope. It is only be added by the identify scope
• Makes it more clear that the email scope is an addition to the identify scope. email does not work / has no function without identify

Checklist

• I have read the contributing guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

[aeneasr]

Awesome 🎉 Thank you for your contribution!