Documentation,cmd/dlv: tidy up --help output (#1649)

* scripts: use relative path in gen-cli-docs.go

Makes gen-cli-docs.go work outside GOPATH.

* Documentation,cmd/dlv: tidy up --help output

The description of --log-dest, --log-output and --backend is very
verbose and messes up the output of --help, move it to two "additional
help" subcommands.

scripts/gen-cli-docs.go

@@ -11,7 +11,7 @@ import (
 )
 
 func main() {
-	fh, err := os.Create(os.ExpandEnv("$GOPATH/src/github.com/go-delve/delve/Documentation/cli/README.md"))
+	fh, err := os.Create(os.ExpandEnv("./Documentation/cli/README.md"))
 	if err != nil {
 		log.Fatalf("could not create README.md: %v", err)
 	}