As you know every Wikimedia project needs its own help for the fundamental topics. The explanation of general topics like wiki-syntax teaching is in every project the same, or you must find it you help topics in the Wikipedia. Why don't create a special Wikimedia project, whith a General help for the topics which belong to every Wiki? Maybe this can created on a way, you can visit and change the Helping page from direct every Wiki, so a User just have to attend it, to make special operations (creating, moving, deleting, protecting and other administrator operations). Of course you can't replace the hole, e.g Wikibooks still needs its instructions about the name conversation or Wikipedia needs "How to write a good article?". But many topics still needn't explained in each topic.
xx stands for the language code
- xx-help.wikimedia.org (this is especially suitable, if you can edit the help direct from the projects.)