Clarify Godep vs GOPATH in documentation

The immediate impetus was that the `restore`
command had unclear help text. Insofar as
reasonable, stick to `go`'s help text.

Note that the verb "check out" in `restore`'s
long help text is used in a similar context in
`go help get`:

"When checking out or updating a package, get
looks for a branch or tag that matches the locally
installed version of Go."

Fixes #33.