Use cleanup_ctx in pool usage doc (MagicStack#878)

Co-authored-by: Elvis Pranskevichus <elvis@edgedb.com>

Author: ir4y

docs/usage.rst

@@ -438,20 +438,26 @@ Web service that computes the requested power of two.
  text="2 ^ {} is {}".format(power, result))
 
 
- async def init_app():
+ async def init_db(app):
+ """Initialize a connection pool."""
+ app['pool'] = await asyncpg.create_pool(database='postgres',
+ user='postgres')
+ yield
+ app['pool'].close()
+
+ 
+ def init_app():
  """Initialize the application server."""
  app = web.Application()
- # Create a database connection pool
- app['pool'] = await asyncpg.create_pool(database='postgres',
- user='postgres')
+ # Create a database context
+ app.cleanup_ctx.append(init_db)
  # Configure service routes
  app.router.add_route('GET', '/{power:\d+}', handle)
  app.router.add_route('GET', '/', handle)
  return app
 
 
- loop = asyncio.get_event_loop()
- app = loop.run_until_complete(init_app())
+ app = init_app()
  web.run_app(app)
 
 See :ref:`asyncpg-api-pool` API documentation for more information.