Hey Rob, you'll find the help files here Nintex Forms Help Files Excerpts (look here for user guide content).
Otherwise Getting Started with Nintex Forms has some beginner content for forms too.
There are a variety of blog posts throughout the site for forms which are more scenario based - have a look in Tech Blog or Learn More (I've filtered the blog posts by "nintex forms"). The Nintex Gallery also has examples of forms and tips and tricks submitted by community members.
Is there a specific question you're after?
I'm trying to link a Choice option in a Nintex Form to Read & Update document actions in Nintex Workflow, so that a final document is updated with data according to the choices made in the initial form. I found this:
which is helpful regarding the general workflow, but I'm finding it difficult to pin down more specific information on how to use Nintex to build out the form.
I don't know the answer to that, but I found the discussion you posted Specify multiple 'read document' choice controls from Nintex Form and have passed it on to one of our Tech evangelists to have a look at.
This is what you are looking for: http://nintexdownload.com/sl/supportfiles/NintexWorkflow2013Helpfile.pdf