net: add note about disabling loopback in ListenMulticastUDP()
net: add note about disabling loopback in ListenMulticastUDP()

Fixes #41752

Change-Id: I83520d2303e5fd2e5f6329f092b40e73c13771a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/271908
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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