cmd/go: add the Retract field to 'go help mod edit' definition of the GoMod struct

Fixes #43281

Change-Id: Ife26ca174a8818b56aaea9547976d97978478a5f
GitHub-Last-Rev: 85a3d30001
GitHub-Pull-Request: golang/go#43315
Reviewed-on: https://go-review.googlesource.com/c/go/+/279592
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>

src/cmd/go/alldocs.go

@@ -1192,6 +1192,7 @@
 // 		Require []Require
 // 		Exclude []Module
 // 		Replace []Replace
+// 		Retract []Retract
 // 	}
 //
 // 	type Require struct {

src/cmd/go/internal/modcmd/edit.go

@@ -95,6 +95,7 @@ writing it back to go.mod. The JSON output corresponds to these Go types:
 		Require []Require
 		Exclude []Module
 		Replace []Replace
+		Retract []Retract
 	}
 
 	type Require struct {