Add notices about disabling auth features for users with 2FA.

Related to #2979 (closed)

• Document the proposed changes to the GitLab authentication system.
• This is done because currently, users with 2FA enabled are allowed API access without a 2FA token.

Tasks

• #2979 (closed) !xxxx - Document proposed auth changes for 2FA users
  • Wait for replies on "potential avenues for documenting the planned changes"
  • Update documentation
  • CHANGELOG entry?
  • Merge conflicts