Fix incorrect security events description for SBFM

[cf-matthias]

Fix incorrect security events description for SBFM

Summary

Fix incorrect security events description for SBFM. This was always displayed under Managed rules, including in the old security dashboard. The distinction can be made by the rule name (and managed rule IDs).

Documentation checklist

• The change adheres to the documentation style guide.

[cloudflare-docs-bot]

Review

✅ No issues found in commit 8e9f75b.

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