Document how key configuration and property configuration interact

How about specifying the primary key that has a different SQL column name than my property name?

I believe the following works:

```csharp
modelBuilder.Entity()
.HasKey(myEntity => myEntity.MyId);

modelBuilder.Entity()
.Property(p => p.MyId)
.HasColumnName("MyCustomColumnNameId");
```

I'm happy to update the docs with that pending confirmation it would be accepted?

Reference: https://stackoverflow.com/a/48155599/185123

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#### Document Details

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* ID: d0a084df-15c4-ab25-26ca-83c8bdf6f026
* Version Independent ID: f1fba5b2-eb2e-da0a-fce0-d492855c1777
* Content: [Keys (primary) - EF Core](https://docs.microsoft.com/en-us/ef/core/modeling/keys)
* Content Source: [entity-framework/core/modeling/keys.md](https://github.com/aspnet/EntityFramework.Docs/blob/master/entity-framework/core/modeling/keys.md)
* Product: **entity-framework**
* Technology: **entity-framework-core**
* GitHub Login: @rowanmiller
* Microsoft Alias: **divega**