Add a comment for ChromeAppCacheService::Unbind.

This is a follow-up to 199e61ec.

Bug: 850921
Change-Id: Icabee6ab7f196b3a59bf3b541aa9bc698f636386
Reviewed-on: https://chromium-review.googlesource.com/1140056Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575607}

content/browser/appcache/chrome_appcache_service.h

@@ -59,8 +59,11 @@ class CONTENT_EXPORT ChromeAppCacheService
   void Bind(std::unique_ptr<mojom::AppCacheBackend> backend,
             mojom::AppCacheBackendRequest request,
             int process_id);
-  // Unbinds the pipe corresponding the to the given process_id.
-  // It does nothing if no pipe was bound.
+  // Unbinds the pipe corresponding to the given process_id. Unbinding
+  // unregisters and destroys the existing backend for that process_id.
+  // The function must be called before a new backend is created for the given
+  // process_id to ensure that there is at most one backend per process_id.
+  // The function does nothing if no pipe was bound.
   void Unbind(int process_id);
 
   void Shutdown();