Document the interaction of autouse scopes

I wouldn't have even attempted what I did to cause #1688 if this had
been there.

Author: quodlibetor

doc/en/fixture.rst

@@ -798,6 +798,10 @@ If we run it, we get two passing tests::
 
 Here is how autouse fixtures work in other scopes:
 
+- autouse fixtures obey the ``scope=`` keyword-argument: if an autouse fixture
+  has ``scope='session'`` it will only be run once, no matter where it is
+  defined. ``scope='class'`` means it will be run once per class, etc.
+
 - if an autouse fixture is defined in a test module, all its test
   functions automatically use it.