Docs: Improve docs around custom notifications

Problem to solve

Our docs for custom notification levels don't mention the actual UI text that's present in the notification settings. I can only see what's available for Custom notifications after I click into the section in my settings, but it would be good to be able to review what's available here in the docs.

We should expand the custom notification settings details in the docs to clarify this, and have the exact UI text listed: https://docs.gitlab.com/user/profile/notifications/

The exact UI text is:

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Further details

Proposal

Who can address the issue

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