Note status of regexes for EmailAddressAttribute, PhoneAttribute, UrlAttribute

## Note status of regexes for EmailAddressAttribute, PhoneAttribute, UrlAttribute

See dotnet/dotnet-api-docs#686

In .NET Framework to version 4.6, the [EmailAddressAttribute](https://docs.microsoft.com/dotnet/api/system.componentmodel.dataannotations.emailaddressattribute), [PhoneAttribute](https://docs.microsoft.com/dotnet/api/system.componentmodel.dataannotations.phoneattribute), and [UrlAttribute](https://docs.microsoft.com/dotnet/api/system.componentmodel.dataannotations.urlattribute) relied on regexes for validation. Version 4.6.1 made it possible to disable regex validation, which was on by default. Version 4.7.2 disabled regex validation by default.

.NET Core does not support regex validation for these types.

This needs to be made explicit in the documentation for the three types. For more information, see ["dataAnnotations:dataTypeAttribute:disableRegex" app setting is on by default in .NET Framework 4.7.2](https://docs.microsoft.com/en-us/dotnet/framework/migration-guide/runtime/4.0-4.7.2#web-applications).

//cc @KyleMit