Update SMTP Troubleshooting Page

What does this MR do?

A customer recently shared that in GitLab 15.11, their emails were not sending. They self-diagnosed and determined that both START TLS and SMTP TLS were enabled on their instance:

gitlab_rails['smtp_enable_starttls_auto'] = true
gitlab_rails['smtp_tls'] = true

In a recent merge request, introduced in GitLab 15.11, there is now a check against if both options have been enabled, and an error is returned accordingly.

This MR updates the SMTP troubleshooting section with the returned error message to provide an indication on why this has occurred, and what should be done to correct the behaviour.

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