At the moment I don't think there is any documentation that highlights SM only APIs. The SM documentation itself is contained within the more generic API doc that you referred to earlier (https://developer.cisco.com/meraki/api-v1/), and you would have to know your way. The important and useful SM endpoints will be under API/Products/sm/ in the API reference menu (get devices, modify tags etc...). You might also find the endpoints under Platform/configure/pii/smDevicesForKey and Platform/configure/pii/smOwnersForKey useful when making queries that involved device owner.
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