Docs feedback: Clarify SSL details

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Follow-up from: https://support.gitlab.com/hc/en-us/requests/111214

Would be great to clarify this section: https://docs.gitlab.com/omnibus/settings/ssl.html#details-on-how-gitlab-and-ssl-work

After this paragraph:

Here we see the fingerprint of the certificate is 7f279c95, which links to the custom certificate.

We should also add:

(Make sure that your customcacert.pem file only contains one certificate! If it's a certificate chain, create individuals files for each in this directory)