Update manage-alerts.md

[PremMS-MDE]

Alert suppression is not compatible and might not work as expected for Custom detection, hence it is not advisable to create suppression rules for custom detection

[learn-build-service-prod]

Learn Build status updates of commit 42cc278:

✅ Validation status: passed

File	Status	Preview URL	Details
defender-endpoint/manage-alerts.md	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit bd1a539:

✅ Validation status: passed

File	Status	Preview URL	Details
defender-endpoint/manage-alerts.md	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[denisebmsft]

Thank you @PremMS-MDE