impl/prod: Make AEContext private.

impl/prod/context_vm.go

Index: impl/prod/context_vm.go
diff --git a/impl/prod/context_vm.go b/impl/prod/context_vm.go
index 874fc90fcefcf148d5a4e88377af2d7f35829659..d97d9ef4bd8c84fa91827e95885fe9e90bed738c 100644
--- a/impl/prod/context_vm.go
+++ b/impl/prod/context_vm.go
@@ -15,6 +15,9 @@ import (
 // implementations which aren't associated with any particular request.
 //
 // This is only available on Managed VMs.
+//
+// It is important to note that this DOES NOT install the AppEngine SDK into the
+// supplied Context. See the warning in Use for more information.
 func UseBackground(c context.Context) context.Context {
 	return setupAECtx(c, appengine.BackgroundContext())
 }