Add comment about why we are returning nil in instantiateViewController method.

Author: selanthiraiyan

WooCommerce/Classes/Authentication/AuthenticationManager.swift

@@ -383,6 +383,9 @@ extension AuthenticationManager: WordPressAuthenticatorDelegate {
         let supportViewController = UIStoryboard.dashboard.instantiateViewController(identifier: identifier,
                                                                                      creator: { coder -> HelpAndSupportViewController? in
             guard let customURL = sourceTag.customHelpCenterURL else {
+                /// Returning nil as we don't need to customise the HelpAndSupportViewController
+                /// In this case `instantiateViewController` method will use the default `HelpAndSupportViewController` created from storyboard.
+                ///
                 return nil
             }