Clarify notification emails docs

What does this MR do?

In the Notification emails doc:

For SaaS:

• Call out that the opt-out in the notifications does not opt users out of all emails. The best case is to make sure that users are clear about what this is turning on/off, and tell them how to opt-out of other emails.

For Self-Hosted

• Say product marketing emails are turned on by default and can be disabled by the administrator.
• The emails are generated and automatically sent by the self-hosted version and sent based on actions of the user. At no time is any personal information sent to GitLab by the tool or by the company by virtue of turning this on.
• Make it clear that turning off these emails only turns of the automatically generated emails from the system, and it will not opt them out of all emails.

Related issues

gitlab-org/growth/team-tasks#364 (closed)

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