runtime: document netpollclose
runtime: document netpollclose

Document that network poller implementations need to define
netpollclose.

Change-Id: Idc73dea7cfd503d4de7e1d95902f0f102cf5ed8f
Reviewed-on: https://go-review.googlesource.com/c/go/+/297809
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,nawawi/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,golang/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,golang/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go