We have launched the client side error logging client and server code. Errors are flowing in and now this needs to be documented so developers know this functionality is available. @Tgr
suggested documenting things this way:
Apart from inline docs, I'd mention the core part at mw:Manual:How to debug#Logging and mw:Help:Locating broken scripts and mw:ResourceLoader/Core modules and mw:Requests for comment/Server-side Javascript error logging, and the Wikimedia-specific part is probably worth its own page on wikitech.