cmd/go: make go tool suggest 'go doc cmd/<command>'

$ gotip tool -h says:

  For more about each tool command, see 'go tool command -h'.

But it's better to suggest

  go doc cmd/<command>

Fixes #18313

Change-Id: I0a36d585906a5e1879e5b7927d1b6173e97cb500
Reviewed-on: https://go-review.googlesource.com/55990Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/alldocs.go

@@ -782,7 +782,7 @@
 // The -n flag causes tool to print the command that would be
 // executed but not execute it.
 //
-// For more about each tool command, see 'go tool command -h'.
+// For more about each tool command, see 'go doc cmd/<command>'.
 //
 //
 // Print Go version

src/cmd/go/internal/tool/tool.go

@@ -27,7 +27,7 @@ With no arguments it prints the list of known tools.
 The -n flag causes tool to print the command that would be
 executed but not execute it.
 
-For more about each tool command, see 'go tool command -h'.
+For more about each tool command, see 'go doc cmd/<command>'.
 `,
 }