net: improve IPMask docs
net: improve IPMask docs

Fixes #28957

Change-Id: Ie8ba841bd4ee71766bcfbbfbdc9173b9be867ed1
Reviewed-on: https://go-review.googlesource.com/c/go/+/151479
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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