Yup, dev documents are autogenerated from comments via luadoc. There're a bunch of undocumented functions.
At this time the best way to learn to write plugins and to hook into things is to have a look at upstream examples.
To add a sort hint have a look at
https://github.com/koreader/koreader...in.lua#L33-L36
> how to add an item in the long press menu in the file picker
Not sure if we have a "shortcut" to address this, like we have to hook on the highlights menu. What's your plan?
There's `DocumentRegistry:addAuxProvider` that lets you handle filetypes in your own ways, but that just adds your implementation to the "Open with" in FM long press menu.