Clarify eager loading of multiple levels using a single .Include()

Clarify the cases in which you can include multiple levels of related entities using a single include statement.
In the Blogs example, instead of using `.ThenInclude` to include the Author's photo object you can go `Blogs.Include(blog => blog.Author.Photo);`.
Specify that this can only be used to navigate through non-collection properties, if that is indeed the case.

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#### Document Details

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* ID: bbd36f35-e257-d0ed-8f28-f539ddcb71ec
* Version Independent ID: 093903d5-f7c7-5a78-c0e5-0e2662dcedef
* Content: [Loading Related Data - EF Core](https://docs.microsoft.com/en-us/ef/core/querying/related-data#feedback)
* Content Source: [entity-framework/core/querying/related-data.md](https://github.com/aspnet/EntityFramework.Docs/blob/master/entity-framework/core/querying/related-data.md)
* Product: **entity-framework**
* Technology: **entity-framework-core**
* GitHub Login: @rowanmiller
* Microsoft Alias: **avickers**