Add further info to SSL instructions on Enterprise Users page

What does this MR do and why?

The phrasing around the Certificate section of the Domain Verification steps is a bit vague, and while it mentions that adding an SSL of your own is optional, it doesn't explicitly state that users should leave the Automatic certificate management using Let's Encrypt option selected if they do not have an SSL or do not wish to supply one. See this Zendesk ticket for an example.

Relates to gitlab-com/support/support-team-meta#5114 (closed)

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