At the moment there isn't a manual for DOCman. If you want to help out (even if you're only submitting one item) you can post in the forum or contact us at support -at- joomlatools -dot- org.
Sources of information:
A good way to learn DOCman is by installing it on a (local) testserver and just play around. DOCman is pretty 'fool-proof': you won't damage your system by doing something wrong .