You are right that for some reason, the PolicyObject Endpoint doesn't exist in the API Documentation on http://developer.cisco.com/meraki.
However, if you browse to the "old API documentation" you can actually find the endpoint there. When you're logged in to your Dashboard, click on Help -> API Docs.
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There, browse down to Policy Objects. Previously they were known as Network Objects, but that API is deprecated. Now it's called PolicyObject - just like the endpoint that you have found yourself.
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It's not optimal that Meraki has two different API Documentations and that they are not alligned. Hopfully that is something, Meraki will get fixed. Perhaps @MeredithW can help with poking the right people?
Anyhow, make sure to keep in mind to check both API Docs - the one at developer.cisco.com, and Meraki's own under the Help tab.
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