Update reference-service-limits.md for custom auth extensions

[soneff]

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Learn Build status updates of commit 0578723:

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[learn-build-service-prod]

Learn Build status updates of commit bc7e2e7:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/external-id/customers/reference-service-limits.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

[Jak-MS]

@csmulligan

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[csmulligan]

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