[DOCS] make an explicit warning not to create clients for each request (#324)

Co-authored-by: Steven Silvester <steve.silvester@mongodb.com>

doc/contributors.rst

@@ -18,3 +18,4 @@ The following is a list of people who have contributed to
 - Steven Silvester
 - Julius Park
 - Doeke Buursma
+- Scott Luu

doc/tutorial-asyncio.rst

@@ -454,11 +454,10 @@ to an :class:`aiohttp.web.Application`:
   :start-after: main-start
   :end-before: main-end
 
-Note that it is a common mistake to create a new client object for every
-request; this comes at a dire performance cost. Create the client
-when your application starts and reuse that one client for the lifetime
-of the process. You can maintain the client by storing a database handle
-from the client on your application object, as shown in this example.
+.. warning::
+   It is a common mistake to create a new client object for every request; this comes at a dire performance cost.
+   Create the client when your application starts and reuse that one client for the lifetime of the process.
+   You can maintain the client by storing a database handle from the client on your application object, as shown in this example.
 
 Visit ``localhost:8080/pages/page-one`` and the server responds "Hello!".
 At ``localhost:8080/pages/page-two`` it responds "Goodbye." At other URLs it

doc/tutorial-tornado.rst

@@ -152,10 +152,11 @@ makes it available to request handlers::
         def get(self):
             db = self.settings['db']
 
-It is a common mistake to create a new client object for every
-request; **this comes at a dire performance cost**. Create the client
-when your application starts and reuse that one client for the lifetime
-of the process, as shown in these examples.
+.. warning::
+    It is a common mistake to create a new client object for every
+    request; **this comes at a dire performance cost**. Create the client
+    when your application starts and reuse that one client for the lifetime
+    of the process, as shown in these examples.
 
 The Tornado :class:`~tornado.httpserver.HTTPServer` class's :meth:`start`
 method is a simple way to fork multiple web servers and use all of your