Update API remarks for RoutingEndpointConventionBuilderExtensions.RequireHost to warn about host spoofing

[david-acker]

AspNetCore.Docs was recently updated to include a warning about port spoofing when using RoutingEndpointConventionBuilderExtensions.RequireHost<TBuilder>.

Issue: dotnet/AspNetCore.Docs#29399
PR: dotnet/AspNetCore.Docs#29463

It would be beneficial for a remark with this warning to be added to the XML docs for RoutingEndpointConventionBuilderExtensions.RequireHost<TBuilder> so it appears in the API documentation.