doc: GOFLAGS environment variable not in 1.11 release notes [1.11 backport]

[gopherbot]

@FiloSottile requested issue #27282 to be considered for backport to the next 1.11 minor release.

@gopherbot please file this for backport against 1.11, so the release notes will get updated.

@andybons please make a cherry-pick CL once your change is merged in master.

[gopherbot]

Change https://golang.org/cl/135496 mentions this issue: [release-branch.go1.11] doc/go1.11, cmd/go: elaborate on new GOFLAGS environment variable

[gopherbot]

Closed by merging f2113d9 to release-branch.go1.11.