Whilst AR may not ship with a PDF manual anymore a wealth of info can be found still in the online user guides.
As there is quite a lot of crossover between versions, info in say the AR5 guide, can still be relevant for AR6.
I admit the online guide pages could do with being updated in places and consolidated in others and sometimes you may have to do a little digging but the Search box will be your friend!
User Guides: https://www.artrage.com/manuals/gett...arted-artrage/
And since it's been mentioned, the Custom Brush: https://www.artrage.com/manuals/cust.../custom-brush/