Document boolean return value of app.setAsDefaultProtocolClient + app.removeAsDefaultProtocolClient

docs/api/app.md

@@ -460,6 +460,8 @@ system. Once registered, all links with `your-protocol://` will be opened with
 the current executable. The whole link, including protocol, will be passed to
 your application as a parameter.
 
+Returns `true` when the call succeeded, otherwise returns `false`.
+
 **Note:** On macOS, you can only register protocols that have been added to
 your app's `info.plist`, which can not be modified at runtime. You can however
 change the file with a simple text editor or script during build time.
@@ -474,6 +476,8 @@ The API uses the Windows Registry and LSSetDefaultHandlerForURLScheme internally
 This method checks if the current executable as the default handler for a
 protocol (aka URI scheme). If so, it will remove the app as the default handler.
 
+Returns `true` when the call succeeded, otherwise returns `false`.
+
 ### `app.isDefaultProtocolClient(protocol)` _macOS_ _Windows_
 
 * `protocol` String - The name of your protocol, without `://`.