Fix documentation and remove sign 'coroutine' where it is deprecated
Fix documentation and remove sign 'coroutine' where it is deprecated

Closes https://github.com/aio-libs/aiohttp/issues/1851.
hellysmile/aiohttp,rutsky/aiohttp,arthurdarcet/aiohttp,playpauseandstop/aiohttp,pfreixes/aiohttp,arthurdarcet/aiohttp,KeepSafe/aiohttp,pfreixes/aiohttp,rutsky/aiohttp,Eyepea/aiohttp,KeepSafe/aiohttp,arthurdarcet/aiohttp,rutsky/aiohttp,hellysmile/aiohttp,KeepSafe/aiohttp