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

[IdeaHunter]

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 dotnet/efcore#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
• Content Source: entity-framework/core/modeling/shadow-properties.md
• Product: entity-framework
• Technology: entity-framework-core
• GitHub Login: @rowanmiller
• Microsoft Alias: divega