net: update BUG sections for go1.12

To confirm this change with the go commaned, please run 'go doc net'
instead of 'go doc -all net'; the -all option surpresses BUG sections.

Change-Id: Iac7bc85fbef48e91d5ede16da0ce4a7ab8cae539
Reviewed-on: https://go-review.googlesource.com/c/157297


Run-TryBot: Mikio Hara <mikioh.public.networking@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>