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

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new