cmd/go: update go help mod edit with the tool and ignore sections

The types were added to the docs, but not the fields in GoMod.
While here, I was initially confused about what is the top-level type
given that `type Module` comes first. Move `type GoMod` to the top
as it is the actual top-level type.

Change-Id: I1270154837501f5c7f5b21959b2841fd4ac808d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/700116
Reviewed-by: Sean Liao <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
Reviewed-by: Junyang Shao <[email protected]>

Author: mvdan

src/cmd/go/alldocs.go

@@ -104,11 +104,6 @@ writing it back to go.mod.
 The -json flag prints the final go.mod file in JSON format instead of
 writing it back to go.mod. The JSON output corresponds to these Go types:
 
-	type Module struct {
-		Path    string
-		Version string
-	}
-
 	type GoMod struct {
 		Module    ModPath
 		Go        string
@@ -118,6 +113,13 @@ writing it back to go.mod. The JSON output corresponds to these Go types:
 		Exclude   []Module
 		Replace   []Replace
 		Retract   []Retract
+		Tool      []Tool
+		Ignore    []Ignore
+	}
+
+	type Module struct {
+		Path    string
+		Version string
 	}
 
 	type ModPath struct {