"Overriding value generation" code sample doesn't work for explicitly setting values

### Type of issue

Code doesn't work

### Description

The "Overriding value generation" code sample doesn't appear to work correctly. When using ValueGeneratedOnAddOrUpdate, it won't save an explicit value. I had to use ValueGeneratedOnAdd instead to get it to auto-generate a value when none supplied and still save explicitly set property values.

See also this guide, which explains how ValueGeneratedOnAddOrUpdate and ValueGeneratedOnAdd behave differently when setting explict values: https://www.learnentityframeworkcore.com/model/generated-value

### Page URL

https://learn.microsoft.com/en-us/ef/core/modeling/generated-properties?tabs=fluent-api

### Content source URL

https://github.com/dotnet/EntityFramework.Docs/blob/main/entity-framework/core/modeling/generated-properties.md

### Document Version Independent Id

ec833af2-275e-981f-689f-8c65edfc00cd

### Article author

@AndriySvyryd