cmd/go: document go env GOMOD
cmd/go: document go env GOMOD

Fixes #26500.

Change-Id: I0a00009ce10f7aab3e6e79f7218307c3008422d1
Reviewed-on: https://go-review.googlesource.com/125655
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

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