add field comments

index.go

@@ -9,8 +9,10 @@ var _ pathWriter = (*ErrIndex)(nil)
 
 // ErrIndex is an error that occurred in a slice. It contains the index of the element.
 type ErrIndex struct {
+	// The index of the slice where the error occurred.
 	Index int
-	Err   error
+	// The underlying error.
+	Err error
 }
 
 // Error returns the index and the error message.

key.go

@@ -9,7 +9,9 @@ var _ pathWriter = (*ErrKey)(nil)
 
 // ErrKey is an error that occurred in a map. It contains the key of the element.
 type ErrKey struct {
+	// The key of the map where the error occurred.
 	Key string
+	// The underlying error.
 	Err error
 }