Edit notification API doc for consistency

What does this MR do?

This MR will improve consistency about version histories and other stuff.

This updates the api directory (only files starting with letter n) in the documentation. See the full progress of this consistency update in !69493 (closed)

Open Questions

• api/notes.md -> in the overview, epic notes are marked as premium, the actual heading is marked as ultimate
• api/notification_settings.md -> Epics are available at premium level, are the notifications ultimate only or is there an error?

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.