Class docs for UrlHelperExtensions are incomplete

I use the `Url.Page` methods from [`UrlHelperExtensions`](https://github.com/dotnet/aspnetcore/blob/610fe433e07b32ed451ed99d07df63be47c29050/src/Mvc/Mvc.Core/src/UrlHelperExtensions.cs). They return `string?`, which trigger analysers in my code (it's the same with the other methods: `Action`, `ActionLink`, `Page`, `PageLink`, `RouteUrl`).

When can those methods return null? The class docs do not say.

(Asked on [SO](https://stackoverflow.com/q/78134624/9971404) without resolution.)