doc/code: mention $GOBIN

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5989045

doc/code.html

@@ -300,7 +300,8 @@ func main() {
 
 <p>
 Next, run <code>go install</code>, which builds and installs the binary to
-<code>$GOPATH/bin</code>:
+<code>$GOPATH/bin</code> (or <code>$GOBIN</code>, if set; to simplify
+presentation, this document assumes <code>GOBIN</code> is unset):
 </p>
 
 <pre>