[MRG+1] Better explain middleware orders and processing directions

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

docs/topics/downloader-middleware.rst

@@ -27,7 +27,11 @@ The :setting:`DOWNLOADER_MIDDLEWARES` setting is merged with the
 :setting:`DOWNLOADER_MIDDLEWARES_BASE` setting defined in Scrapy (and not meant
 to be overridden) and then sorted by order to get the final sorted list of
 enabled middlewares: the first middleware is the one closer to the engine and
-the last is the one closer to the downloader.
+the last is the one closer to the downloader. In other words,
+the :meth:`~scrapy.downloadermiddlewares.DownloaderMiddleware.process_request`
+method of each middleware will be invoked in increasing
+middleware order (100, 200, 300, ...) and the :meth:`~scrapy.downloadermiddlewares.DownloaderMiddleware.process_response` method
+of each middleware will be invoked in decreasing order.
 
 To decide which order to assign to your middleware see the
 :setting:`DOWNLOADER_MIDDLEWARES_BASE` setting and pick a value according to

docs/topics/spider-middleware.rst

@@ -28,7 +28,12 @@ The :setting:`SPIDER_MIDDLEWARES` setting is merged with the
 :setting:`SPIDER_MIDDLEWARES_BASE` setting defined in Scrapy (and not meant to
 be overridden) and then sorted by order to get the final sorted list of enabled
 middlewares: the first middleware is the one closer to the engine and the last
-is the one closer to the spider.
+is the one closer to the spider. In other words,
+the :meth:`~scrapy.spidermiddlewares.SpiderMiddleware.process_spider_input`
+method of each middleware will be invoked in increasing
+middleware order (100, 200, 300, ...), and the
+:meth:`~scrapy.spidermiddlewares.SpiderMiddleware.process_spider_output` method
+of each middleware will be invoked in decreasing order.
 
 To decide which order to assign to your middleware see the
 :setting:`SPIDER_MIDDLEWARES_BASE` setting and pick a value according to where