cmd/go: document GoVersion field in Module struct

The 'go version' statement was added during Go 1.11 development in
CL 125940. That CL added the GoVersion field to modinfo.ModulePublic
struct, but did not document it in cmd/go documentation. This was
consistent with the CL description, which stated "We aren't planning
to use this or advertise it much yet".

CL 147281, applied during Go 1.12 development, was a change to start
adding the 'go version' statement when initializing go.mod. The 'go
version' statement is now being used, and it has been documented in
the Go 1.12 release notes at https://golang.org/doc/go1.12#modules.
It's now due time to documement the GoVersion field in cmd/go as well.

Keep the Error field bottom-most, both because it makes sense not to
place it in the middle of other fields, and for consistency with the
field order in struct Package, where the Error information is located
at the very bottom.

Regenerate alldocs.go by running mkalldocs.sh.

Updates golang#28221
Updates golang#23969

Change-Id: Iaf43a0da4f6a2489d861092a1d4e002a532952cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/164878
Run-TryBot: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

src/cmd/go/internal/list/list.go

@@ -202,17 +202,18 @@ When listing modules, the -f flag still specifies a format template
 applied to a Go struct, but now a Module struct:
 
     type Module struct {
-        Path     string       // module path
-        Version  string       // module version
-        Versions []string     // available module versions (with -versions)
-        Replace  *Module      // replaced by this module
-        Time     *time.Time   // time version was created
-        Update   *Module      // available update, if any (with -u)
-        Main     bool         // is this the main module?
-        Indirect bool         // is this module only an indirect dependency of main module?
-        Dir      string       // directory holding files for this module, if any
-        GoMod    string       // path to go.mod file for this module, if any
-        Error    *ModuleError // error loading module
+        Path      string       // module path
+        Version   string       // module version
+        Versions  []string     // available module versions (with -versions)
+        Replace   *Module      // replaced by this module
+        Time      *time.Time   // time version was created
+        Update    *Module      // available update, if any (with -u)
+        Main      bool         // is this the main module?
+        Indirect  bool         // is this module only an indirect dependency of main module?
+        Dir       string       // directory holding files for this module, if any
+        GoMod     string       // path to go.mod file for this module, if any
+        GoVersion string       // go version used in module
+        Error     *ModuleError // error loading module
     }
 
     type ModuleError struct {

src/cmd/go/internal/modinfo/info.go

@@ -20,8 +20,8 @@ type ModulePublic struct {
 	Indirect  bool          `json:",omitempty"` // module is only indirectly needed by main module
 	Dir       string        `json:",omitempty"` // directory holding local copy of files, if any
 	GoMod     string        `json:",omitempty"` // path to go.mod file describing module, if any
-	Error     *ModuleError  `json:",omitempty"` // error loading module
 	GoVersion string        `json:",omitempty"` // go version used in module
+	Error     *ModuleError  `json:",omitempty"` // error loading module
 }
 
 type ModuleError struct {