Add info about limitation/differences between 'shadow property' vs 'shadow navigation'

There is one thing needed to be highlighted in documentation is limitation/misconception of shadow properties:
Right now if you read documentation it may give you impression that **any** kind of property that doesn't exists on clr type can be a shadow property, but this isn't true - shadow property can't be(_or at least its not yet supported, see aspnet/EntityFrameworkCore#3864 and aspnet/EntityFrameworkCore#15655_) a property that points out to a different model aka navigation property.

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#### Document Details

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* ID: 550a3c36-7f1d-efa7-3ed8-769720168da4
* Version Independent ID: 627c4301-e00d-826e-d545-3a244305dec9
* Content: [Shadow Properties - EF Core](https://docs.microsoft.com/en-us/ef/core/modeling/shadow-properties)
* Content Source: [entity-framework/core/modeling/shadow-properties.md](https://github.com/aspnet/EntityFramework.Docs/blob/master/entity-framework/core/modeling/shadow-properties.md)
* Product: **entity-framework**
* Technology: **entity-framework-core**
* GitHub Login: @rowanmiller
* Microsoft Alias: **divega**