Write a topic about on_startup/on_shutdown signals usage for handling objects like DB connections

[asvetlov]

Now http://aiohttp.readthedocs.io/en/stable/web.html has signals chapter but it mentions on_prepare only: http://aiohttp.readthedocs.io/en/stable/web.html#signals
There are another topics about graceful shutdown: http://aiohttp.readthedocs.io/en/stable/web.html#graceful-shutdown and background tasks: http://aiohttp.readthedocs.io/en/stable/web.html#background-tasks

But we need explicit topic for pointing on redis/postgres/whatever initialization/cleaning-up.

See also https://stackoverflow.com/questions/45134300/aioredis-aiohttp-proper-usage-of-connection-pooling

[asvetlov]

Done by #2196

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