Update return values of basic-auth.access:retrieve_credentials

[steveoliver]

Update the docblock of basic-auth.access:retrieve_credentials to reflect the username and password values returned.

Summary

Checklist

• The Pull Request has tests (N/A?)
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE (N/A?)

Issue reference

Fix #[issue number]

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