Hi kwiz
No, I never got any answer on this. But I figured it out on my own, eventually.
I never got down to the bottom of the entire Max documentation because that would have involved figuring out the Max XML syntax in its entirety. And even Cycling seem to use their own dedicated tool that just fills their XML files with content from another place/database. But I got my object's original Max 5 documentation working again. I am not too sure anymore but I think I did not have to change anything relevant in the reference (and tutorial) file headers. While Max 5 at startup seems to crawl through its search paths to index documentation files, Max 6 comes with two files that hold all meta information about the documentation, i.e. reference files, tutorial files (not patchers), vignettes, etc. So, you will have to add some information there, as well.
The two files are: obj-qlookup.json, which lists all objects in form of an XML lookup table, and doc-qlookup.json, which does the same for documents. Both files reside inside the /Cycling 74/Interfaces folder (on a Mac, don't know about Windoze).
The files are quite readable and pretty self explanatory, just stick to the syntax. Besides the brackets keep an eye especially on the commas. Missing a comma or putting a comma in the wrong place makes the whole documentation unusable!
obj-qlookup.json: It's a good thing to put your object(s) into a suitable object category. Doing so makes them easy to spot in the documentation Objects tab and also in the object palette. The thing I did not figure out is the "module" tag. There is also some "module" stuff going on inside the /patches/docs/refpages folder. I tried to mess around with it for a while but figured I did not need it in the end. The price is, I guess, that I cannot have my own refpages folder and have to use the existing max-ref, msp-ref, etc. folder hierarchy. But I can live with that.
doc-qlookup.json: Be sure to put your files inside the correct tags. There are tags like refpages for reference files which branches down into msp-ref, max-ref, etc. The content of a "digest" tag is shown in the documentation Objects tab and search results, maybe elsewhere as well – I did not check further than that. Then there is also a tutorials tag in case you wrote your own tutorials. I was not so sure about its "name" and "title" tags, so I just filled in the same text to be on the safe side.
I guess everybody reading this with the intent to try it at home will already know the dangers ahead. But anyways, here comes the disclaimer: **** Messing around with these files could seriously damage your Max 6 documentation. *** So keep backup copies in case anything goes south!
What's left to say...? Well, I hope someday in the near future we will see Cycling revealing the real magic behind all of this. Maybe even giving out their/some tool to create the documentation in a friendly way. I think EVERYONE would benefit from that, users, programmers, Cycling. What is especially very very bad, is that you cannot combine two 3rd party documentations unless you get the users to do some handiwork themselves. So, if anyone with some more reverse engineering talent than me figures out how to add your own object documentation without needing to overwrite original files, I'll send you flowers. Or a toggle. Or a bang.
Good luck!