IDENTITY_INSERT requires ALTER permissions

While running down an error, I discovered that a user must own the table or have ALTER permission on the table to execute SET IDENTITY_INSERT. I think this is worth noting in the document.

Error:
`Cannot find the object "dbo.Table" because it does not exist or you do not have permissions.`

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#### Document Details

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* ID: e621a525-7104-74bf-5a17-9f5761f75fab
* Version Independent ID: 3d0e675b-3b2a-ebd9-273b-312c5c6ac092
* Content: [Setting Explicit Values for Generated Properties - EF Core](https://docs.microsoft.com/en-us/ef/core/saving/explicit-values-generated-properties)
* Content Source: [entity-framework/core/saving/explicit-values-generated-properties.md](https://github.com/aspnet/EntityFramework.Docs/blob/master/entity-framework/core/saving/explicit-values-generated-properties.md)
* Product: **entity-framework**
* GitHub Login: @rowanmiller
* Microsoft Alias: **divega**