Docentric Designer is actually MS Word with a special Add-In whereas report templates (aka designs or layouts) are just ordinary Word documents. This also means that you can edit report templates (their static content such as colors, fonts, formatting, text, etc.) using plain MS Word.
Docentric report templates used for Dynamics AX 2012 and D365FO are the same (100% compatible) as well as all designing techniques. The same goes for the rest of Docentric AX report artifacts such as DSP classes. That is why you can migrate all Docentric reports from AX 2012 to D365FO solution with almost no effort.
Therefore, Docentric Designer documentation is the same for both AX 2012 and D365FO
The complete documentation you can find in How-To Manuals for AX 2012
Docentric Designer Documentation >>