doc: add auth_secret_file for smtp in document

[nutmos]

Pull Request Checklist

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Which user-facing changes does this PR introduce?

Add smtp_auth_secret_file in global config and auth_secret_file in email config

PR #3087 added smtp_auth_secret_file in global config and auth_secret_file in email config but the document hasn't been updated yet. This PR updates the document for these parameters.

[SoloJacobs]

Thanks, LGTM