Update documentation of ReauthTabHelper::CreateForWebContents

Bug: None
Change-Id: I5029dd1ed6fba56104ae11da8e17710567247ac9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2154950Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/master@{#760434}

chrome/browser/signin/reauth_tab_helper.h

@@ -22,7 +22,8 @@ class ReauthTabHelper : public content::WebContentsUserData<ReauthTabHelper>,
   using ReauthCallback = base::OnceCallback<void(signin::ReauthResult)>;
 
   // Creates a new ReauthTabHelper and attaches it to |web_contents|. If an
-  // instance is already attached, does nothing.
+  // instance is already attached, no replacement happens, just notifies the
+  // caller by invoking |callback| with signin::ReauthResult::kCancelled.
   // Initializes a helper with:
   // - |callback| to be called when the reauth flow is complete.
   // - |reauth_url| that should be the final destination of the reauth flow.