Arguments to table.grow are about to change order

[lars-t-hansen]

SpiderMonkey implemented table.grow before there was a spec, but now there's a spec and it has the order of arguments reversed from us. A patch is about to land in SpiderMonkey that will implement that change. Once it has landed, we must update the documentation here. I may or may need to rev the gc_feature_opt_in version; not sure yet.