Yes the part of the old faqs are under what question can I ask in the help center. Getting the documentation fully integerated and reproduced on the new site is a work in progress. Specifically, that top-level tags are not documented anywhere very visible now, is something I find problematic.
To more directlly answer you question, I think the top-levle tags are not really docuemnted anywhere.
It used to be mainly on the page were one asked questions as some modification of the general software. This is was lost. Maybe it is somewhere in other documentation; but not visble at all.
@SébastienPalcoux no very particular opinion. François G. Dorais's answer seems good to me. Personally, I am not too fond of ideas to generalise MO to a collabortaive tool. At least the main site should rather stay free of this.
