Suggest change to clarify access modifier required when using Lazy Loading Proxies

When combining entity type constructors with lazy-loading proxies (enabled via a call to `UseLazyLoadingProxies(true)`), model constructors must be at a minimum `protected`, or exceptions will be thrown.

Suggest changing this line:
> The constructor can be public, private, or have any other accessibility.

to:
> The constructor can be public, private, or have any other accessibility. However when using Lazy Loading Proxies the constructor must be, at minimum, protected.

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#### Document Details

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* ID: ecc7aedf-6301-7ec7-4f65-81749b1f54c8
* Version Independent ID: d0774ffa-c841-e6df-72de-d7f53c0d6f6f
* Content: [Entity types with constructors - EF Core](https://docs.microsoft.com/en-us/ef/core/modeling/constructors)
* Content Source: [entity-framework/core/modeling/constructors.md](https://github.com/aspnet/EntityFramework.Docs/blob/master/entity-framework/core/modeling/constructors.md)
* Product: **entity-framework**
* GitHub Login: @ajcvickers
* Microsoft Alias: **divega**