os: reference LookupEnv from the Getenv docs

Fixes #16782

Change-Id: If54917bf5ca1588d8a6d443c3aa6e1d4ada6b620
Reviewed-on: https://go-review.googlesource.com/27322
Reviewed-by: anatoly techtonik <techtonik@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/env.go

@@ -76,6 +76,7 @@ func getShellName(s string) (string, int) {
 
 // Getenv retrieves the value of the environment variable named by the key.
 // It returns the value, which will be empty if the variable is not present.
+// To distinguish between an empty value and an unset value, use LookupEnv.
 func Getenv(key string) string {
 	v, _ := syscall.Getenv(key)
 	return v