feat: add information about flow changing from register to login and …

[abador]

…the other way around, SPLAT-682

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Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
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• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
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• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

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