Document ink, or document-proof ink, is usually seen in German ink brands - it's ink made to meet German standards for record keeping. Elsewhere in the world, it may not be enforced as a standard, but 'document ink' should generally mean something permanent, that won't fade, and will remain readable if it gets wet.
Most of these will need a little extra care when used in fountain pens, but should be perfectly safe if they're not left to dry - not what we'd recommend for any especially valuable pens, but very good for day to day use.
