I am with you that documentation is VERY difficult to find on the Nintex site(s). The page I linked to has a link to http://nintexdownload.com/helpFiles/NF2013/NintexForms2013Help.html which has a link 3.20 Runtime Functions and 3.21 Lookup function. I don't know if these give you what you need.
Some other tidbits I have found,
http://help.nintex.com - can't even remember how I found that link. I don't see it on the site anywhere. Seems to only have O365 stuff and some nintex mobile. Maybe most of the docs apply to on-prem as well. Also has links to SDKs.
http://nintexdownload.com/helpFiles/NF2013/NintexForms2013Help.html - again I don't know how I found this. Google?
(just for the heck of it I tried following the naming pattern as http://nintexdownload.com/HelpFiles/NW2010/NintexWorkflow2010Help.html, but that does not exist)
NW 2010 PDF (I swear I used to have the HTML version bookmarked, but now can't find it)
(as above I tried http://nintexdownload.com/sl/supportfiles/NintexWorkflow2013UserManual.pdf, but no dice)
A comment here https://community.nintex.com/message/30829#comment-30829 says "Where have you been all my life?!?! I've been needing this reference documentation for years! Thanks so much for the helpful explanations, examples, and notifications of what is obsolete." So you and I are not alone by any means...