Incorrect CustomValidationAttribute remarks

Content: https://docs.microsoft.com/en-us/dotnet/api/system.componentmodel.dataannotations.customvalidationattribute.method?view=netcore-2.2
Content source: [xml/System.ComponentModel.DataAnnotations/CustomValidationAttribute.xml](https://github.com/dotnet/docs/blob/master/xml/System.ComponentModel.DataAnnotations/CustomValidationAttribute.xml)

The remarks for the `Method` property of `CustomValidationAttribute` incorrectly state that the specified method must return a `Boolean` and takes a `ValidationResult` `out` param. In .Net Core 2.2 at least, it must return a `ValidationResult`.

Maybe the signature is different in different languages?

Additionally, it would be preferable to have the valid signatures listed as code snippets rather than described in text.