Mention an Example of Optional Relationship

In the section on Required and Optional Relationships where you show the IsRequired() method, it might help newbies to explain the different effect of required and optional by stating something like "required would mean that a post can't exist without a blog and optional would mean that a post can exist on its own, without being tied to a blog." Also that this only affects EF COre persistence, not your business logic.

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* ID: 24b2b50e-35f8-9e0b-8ce5-840e1ea816e6
* Version Independent ID: 9e0e6e2c-48fa-76c3-3bd9-813a6df22645
* Content: [Relationships - EF Core](https://docs.microsoft.com/en-us/ef/core/modeling/relationships?tabs=fluent-api%2Cfluent-api-simple-key%2Csimple-key#required-and-optional-relationships)
* Content Source: [entity-framework/core/modeling/relationships.md](https://github.com/dotnet/EntityFramework.Docs/blob/master/entity-framework/core/modeling/relationships.md)
* Product: **entity-framework**
* Technology: **entity-framework-core**
* GitHub Login: @AndriySvyryd
* Microsoft Alias: **avickers**