Document changing settings in Spider.from_crawler().

docs/topics/settings.rst

@@ -98,6 +98,26 @@ and settings set there should use the "spider" priority explicitly:
             super().update_settings(settings)
             settings.set("SOME_SETTING", "some value", priority="spider")
 
+.. versionadded:: VERSION
+
+It's also possible to modify the settings in the
+:meth:`~scrapy.Spider.from_crawler` method, e.g. based on :ref:`spider
+arguments <spiderargs>` or other logic:
+
+.. code-block:: python
+
+    import scrapy
+
+
+    class MySpider(scrapy.Spider):
+        name = "myspider"
+
+        @classmethod
+        def from_crawler(cls, crawler, *args, **kwargs):
+            spider = super().from_crawler(crawler, *args, **kwargs)
+            spider.settings.set("SOME_SETTING", kwargs["value"], priority="spider")
+            return spider
+
 3. Project settings module
 --------------------------
 

docs/topics/spiders.rst

@@ -136,6 +136,14 @@ scrapy.Spider
        attributes in the new instance so they can be accessed later inside the
        spider's code.
 
+       .. versionchanged:: VERSION
+
+           The settings available in this method can now be modified, which is
+           handy if you want to modify them based on arguments. As a
+           consequence, the settings available in this method aren't the final
+           values as they can be modified later by e.g. :ref:`add-ons
+           <topics-addons>`.
+
        :param crawler: crawler to which the spider will be bound
        :type crawler: :class:`~scrapy.crawler.Crawler` instance