Add clarification on invoking Promise-based functions via JS interop (#20031)

Author: Safia Abdalla

aspnetcore/blazor/call-javascript-from-dotnet.md

@@ -21,6 +21,8 @@ This article covers invoking JavaScript functions from .NET. For information on
 
 To call into JavaScript from .NET, use the <xref:Microsoft.JSInterop.IJSRuntime> abstraction. To issue JS interop calls, inject the <xref:Microsoft.JSInterop.IJSRuntime> abstraction in your component. <xref:Microsoft.JSInterop.IJSRuntime.InvokeAsync%2A> takes an identifier for the JavaScript function that you wish to invoke along with any number of JSON-serializable arguments. The function identifier is relative to the global scope (`window`). If you wish to call `window.someScope.someFunction`, the identifier is `someScope.someFunction`. There's no need to register the function before it's called. The return type `T` must also be JSON serializable. `T` should match the .NET type that best maps to the JSON type returned.
 
+JavaScript functions that return a [Promise](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Promise) are called with <xref:Microsoft.JSInterop.IJSRuntime.InvokeAsync%2A>. `InvokeAsync` unwraps the Promise and returns the value awaited by the Promise.
+
 For Blazor Server apps with prerendering enabled, calling into JavaScript isn't possible during the initial prerendering. JavaScript interop calls must be deferred until after the connection with the browser is established. For more information, see the [Detect when a Blazor Server app is prerendering](#detect-when-a-blazor-server-app-is-prerendering) section.
 
 The following example is based on [`TextDecoder`](https://developer.mozilla.org/docs/Web/API/TextDecoder), a JavaScript-based decoder. The example demonstrates how to invoke a JavaScript function from a C# method that offloads a requirement from developer code to an existing JavaScript API. The JavaScript function accepts a byte array from a C# method, decodes the array, and returns the text to the component for display.