docs: clarify that 'second-instance' is only emitted when the second …

…instance calls `app.requestSingleInstanceLock()` (#18384)

docs/api/app.md

@@ -390,8 +390,10 @@ Returns:
 * `workingDirectory` String - The second instance's working directory
 
 This event will be emitted inside the primary instance of your application
-when a second instance has been executed. `argv` is an Array of the second instance's
-command line arguments, and `workingDirectory` is its current working directory. Usually
+when a second instance has been executed and calls `app.requestSingleInstanceLock()`.
+
+`argv` is an Array of the second instance's command line arguments,
+and `workingDirectory` is its current working directory. Usually
 applications respond to this by making their primary window focused and
 non-minimized.