Clarify execution order in middleware docs (#1475)

Author: rgroothuijsen

docs/execution/middleware.rst

@@ -41,6 +41,8 @@ And then execute it with:
 
     result = schema.execute('THE QUERY', middleware=[AuthorizationMiddleware()])
 
+If the ``middleware`` argument includes multiple middlewares,
+these middlewares will be executed bottom-up, i.e. from last to first.
 
 Functional example
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