Detect and warn if $(window).on("load"...) is done in a ready handler

[dmethvin]

Per discussion in meeting and jquery/jquery#3194

[dmethvin]

I think this can be generalized to issue a warning if $(window).on("load"...) runs and document.readyState === "complete" since that would mean it is too late to listen for the load event, the tree has already fallen in the forest and you were elsewhere.