We relay this info in a few places, in a few different ways, but not
consistently everywhere.  This led one of our users to start googling
and not find https://golang.org/doc/code.html#Workspaces, of which `go
help gopath` is the most equivalent.

Change-Id: I28a94375739f3aa4f200e145293ca2a5f65101e1
Reviewed-on: https://go-review.googlesource.com/28690
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>