SimpleTypeModelBinder.CheckModel: add API doc

[yecril71pl]

SimpleTypeModelBinder.CheckModel: add API doc

• You've read the Contributor Guide and Code of Conduct.
• You've included unit or integration tests for your change, where applicable.
• You've included inline docs for your change, where applicable.
• There's an open issue for the PR that you are making. If you'd like to propose a new feature or change, please open an issue to discuss the change or find an existing issue.

Description

There is no documentation to inherit from, so specific documentation must be provided.

Having this interface documented would allow me to do the following trick:

protected override
void CheckModel (ModelBindingContext ctx, ValueProviderResult r, object? m)
 { if (m is string v) ctx .Result = ModelBindingResult .Success (v .Trim ()); }