Update IModelBinder documentation

[NTaylorMullen]

Part of its return documentation currently reads:

/// <para>
/// A <see cref="Task"/> which on completion returns a <see cref="ModelBindingResult"/> which
/// represents the result of the model binding process.
/// </para>

However, the signature for the method is: Task BindModelAsync(ModelBindingContext bindingContext);

[rynowak]

@Eilon 1.0.0 approval please