os: Getenv documentation update to point user to os.LookupEnv

[techtonik]

go 1.5 adds os.LookupEnv (#9676)

But it is not sufficiently described on the web, e.g.
https://gobyexample.com/environment-variables

It will valuable to add reference to it to os.Getenv
documentation https://golang.org/pkg/os/#Getenv

[bradfitz]

https://go-review.googlesource.com/27322

[techtonik]

Thanks. I would fix it myself if that was possible to edit on GitHub, but setting up the whole toolchain with Gerrit will probably take too much time for such a small change.

[gopherbot]

CL https://golang.org/cl/27322 mentions this issue.