cmd/digraph: improve examples using go list, mod

Change-Id: Ib04b11b32f40f1602eb4cf0837e331b77511b6c5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/502875
gopls-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Bypass: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

cmd/digraph/digraph.go

@@ -63,18 +63,27 @@ shirt -> sweater, not shirt -> tie -> sweater.
 
 Example usage:
 
-Using digraph with existing Go tools:
+Show which clothes (see above) must be donned before a jacket:
 
-	$ go mod graph | digraph nodes # Operate on the Go module graph.
-	$ go list -m all | digraph nodes # Operate on the Go package graph.
+	$ digraph reverse jacket
 
-Show the transitive closure of imports of the digraph tool itself:
+Many tools can be persuaded to produce output in digraph format,
+as in the following examples.
 
-	$ go list -f '{{.ImportPath}} {{join .Imports " "}}' ... | digraph forward golang.org/x/tools/cmd/digraph
+Using an import graph produced by go list, show a path that indicates
+why the gopls application depends on the cmp package:
 
-Show which clothes (see above) must be donned before a jacket:
+	$ go list -f '{{.ImportPath}} {{join .Imports " "}}' -deps golang.org/x/tools/gopls |
+		digraph somepath golang.org/x/tools/gopls github.com/google/go-cmp/cmp
 
-	$ digraph reverse jacket
+Show which packages in x/tools depend, perhaps indirectly, on the callgraph package:
+
+	$ go list -f '{{.ImportPath}} {{join .Imports " "}}' -deps golang.org/x/tools/... |
+		digraph reverse golang.org/x/tools/go/callgraph
+
+Using a module graph produced by go mod, show all dependencies of the current module:
+
+	$ go mod graph | digraph forward $(go list -m)
 */
 package main // import "golang.org/x/tools/cmd/digraph"