Make architecture overview references a little more clear on the docs

Expliciting what actually happens by adding links to the respective methods
that are invoked in each processing phase.

docs/topics/architecture.rst

@@ -41,25 +41,26 @@ this:
 
 4. The :ref:`Engine <component-engine>` sends the Requests to the
    :ref:`Downloader <component-downloader>`, passing through the
-   :ref:`Downloader Middleware <component-downloader-middleware>`
-   (requests direction).
+   :ref:`Downloader Middlewares <component-downloader-middleware>` (see
+   :meth:`~scrapy.downloadermiddlewares.DownloaderMiddleware.process_request`).
 
 5. Once the page finishes downloading the
    :ref:`Downloader <component-downloader>` generates a Response (with
    that page) and sends it to the Engine, passing through the
-   :ref:`Downloader Middleware <component-downloader-middleware>`
-   (response direction).
+   :ref:`Downloader Middlewares <component-downloader-middleware>` (see
+   :meth:`~scrapy.downloadermiddlewares.DownloaderMiddleware.process_response`).
 
 6. The :ref:`Engine <component-engine>` receives the Response from the
    :ref:`Downloader <component-downloader>` and sends it to the
    :ref:`Spider <component-spiders>` for processing, passing
-   through the :ref:`Spider Middleware <component-spider-middleware>`
-   (input direction).
+   through the :ref:`Spider Middleware <component-spider-middleware>` (see
+   :meth:`~scrapy.spidermiddlewares.SpiderMiddleware.process_spider_input`).
 
 7. The :ref:`Spider <component-spiders>` processes the Response and returns
    scraped items and new Requests (to follow) to the
    :ref:`Engine <component-engine>`, passing through the
-   :ref:`Spider Middleware <component-spider-middleware>` (output direction).
+   :ref:`Spider Middleware <component-spider-middleware>` (see
+   :meth:`~scrapy.spidermiddlewares.SpiderMiddleware.process_spider_output`).
 
 8. The :ref:`Engine <component-engine>` sends processed items to
    :ref:`Item Pipelines <component-pipelines>`, then send processed Requests to