Document rate limit on instance audit events API

What does this MR do?

Cross-references the audit_events_api_limit rate limit (merged in !248055 (merged)) from the existing docs so it is discoverable:

• doc/api/audit_events.md - note on the instance audit events section
• doc/api/settings.md - attribute row and the Premium/Ultimate parameter lists
• doc/rate_limits/api/_index.md - entry in the configurable API rate limits table (this list originally lived in doc/security/rate_limits.md, which became a redirect during the docs reorganization)

The rate limit page itself shipped with the code MR at doc/administration/settings/rate-limit-on-audit-events-api.md; moving it into the new doc/rate_limits/api/ structure is left to a follow-up so this MR stays a set of cross-references.

Related issues

• Related to https://gitlab.com/gitlab-org/gitlab/-/issues/605428

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