[release-branch.go1.11] doc/go1.11, cmd/go: elaborate on new GOFLAGS environment variable

In Go 1.11, cmd/go gained support for the GOFLAGS environment variable.
It was added and described in detail in CL 126656.
Mention it in the Go 1.11 release notes, link to the cmd/go documentation,
and add more details there.

Fixes #27387.

Change-Id: Ifc35bfe3e0886a145478d36dde8e80aedd8ec68e
Reviewed-on: https://go-review.googlesource.com/135035


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Rob Pike <r@golang.org>
(cherry picked from commit 58c6afe0)
Reviewed-on: https://go-review.googlesource.com/135496


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>