cmd/go: add link to env varible guide to set custom GOPATH

Also moves the GOPATH env variable guide to
golang.org/wiki/SettingGOPATH.

Fixes #18294.

Change-Id: I88a2ce550df7466f8d2388d86bc8476dcf3c2ad6
Reviewed-on: https://go-review.googlesource.com/34918Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/code.html

@@ -166,7 +166,7 @@ To learn more about the <code>GOPATH</code> environment variable, see
 
 <p>
 To use a custom workspace location,
-<a href="https://golang.org/wiki/EnvVariables#gopath">set the <code>GOPATH</code> environment variable</a>.
+<a href="https://golang.org/wiki/SettingGOPATH">set the <code>GOPATH</code> environment variable</a>.
 </p>
 
 <h3 id="ImportPaths">Import paths</h3>

src/cmd/go/alldocs.go

@@ -935,6 +935,8 @@
 // unless that directory holds a Go distribution.
 // Run "go env GOPATH" to see the current GOPATH.
 //
+// See https://golang.org/wiki/SettingGOPATH to set a custom GOPATH.
+//
 // Each directory listed in GOPATH must have a prescribed structure:
 //
 // The src directory holds source code.  The path below src

src/cmd/go/help.go

@@ -295,6 +295,8 @@ to a subdirectory named "go" in the user's home directory
 unless that directory holds a Go distribution.
 Run "go env GOPATH" to see the current GOPATH.
 
+See https://golang.org/wiki/SettingGOPATH to set a custom GOPATH.
+
 Each directory listed in GOPATH must have a prescribed structure:
 
 The src directory holds source code.  The path below src