doc/go1.12: new go line in go.mod can break builds with Go 1.11 - 1.11.3
doc/go1.12: new go line in go.mod can break builds with Go 1.11 - 1.11.3

Fixes #30446

Change-Id: If069f72fa9735f839df92f3ede3bf7b6d7a695a5
Reviewed-on: https://go-review.googlesource.com/c/164317
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

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