cue: update Merge comment to REALLY NOT USE IT

Change-Id: I28bf54d49cd43bee54b00526c8d26e7f32105fcd
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9341
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>

cue/instance.go

@@ -278,7 +278,7 @@ func (inst *Instance) Eval(expr ast.Expr) Value {
 	return newValueRoot(ctx, result)
 }
 
-// Merge unifies the given instances into a single one.
+// DO NOT USE.
 //
 // Deprecated: do not use.
 func Merge(inst ...*Instance) *Instance {