go/ast/edge: update Kind const doc comments

- add godoc to explain the enum name syntax
- remove the outdated Kind instability comment

Change-Id: I09ea4efab1631bc46f740d2e2bb5546a6a6a6961
Reviewed-on: https://go-review.googlesource.com/c/tools/+/770480
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Madeline Kalil <mkalil@google.com>
Reviewed-by: Madeline Kalil <mkalil@google.com>

go/ast/edge/edge.go

diff --git a/go/ast/edge/edge.go b/go/ast/edge/edge.go
index 4f6ccfd..4c1e61a 100644
--- a/go/ast/edge/edge.go
+++ b/go/ast/edge/edge.go
@@ -12,7 +12,7 @@
 	"reflect"
 )
 
-// A Kind describes a field of an ast.Node struct.
+// A Kind describes a field of an [ast.Node] struct.
 type Kind uint8
 
 // String returns a description of the edge kind.
@@ -49,13 +49,14 @@
 	return v.Interface().(ast.Node) // may be nil
 }
 
+// Each [Kind] is named Type_Field, where Type is the
+// [ast.Node] struct type and Field is the name of the field
 const (
 	Invalid Kind = iota // for nodes at the root of the traversal
 
-	// Kinds are sorted alphabetically.
-	// Numbering is not stable.
-	// Each is named Type_Field, where Type is the
-	// ast.Node struct type and Field is the name of the field
+	// As of Go1.26 these kinds are sorted alphabetically, but
+	// numbering must be stable, so any new addition of const should
+	// use a new value (be added at the end of the list).
 
 	ArrayType_Elt
 	ArrayType_Len