Missing information that `Validator.ValidateObject` / `Validator.TryValidateObject` also calls `IValidatableObject.Validate`

### Type of issue

Missing information

### Description

From the source code of `Validator.ValidateObject` / `Validator.TryValidateObject` we can see they will first validate all the attributes, then if the given object instance are `IValidatableObject`, it will call the `IValidatableObject.Validate` to validate it.

I believe it's quite important to make this behavior explicit as people may already rely on this behavior for a long time.

### Page URL

https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.dataannotations.validator.validateobject?view=net-9.0

### Content source URL

https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System.ComponentModel.DataAnnotations/Validator.xml

### Document Version Independent Id

6ebc3f46-2534-6c8d-797c-3ec39f57feca

### Article author

@dotnet-bot