Despite what some may say, some level of documentation is needed in Agile. Agile is not an excuse to be a cowboy.

Having said that, not being a cowboy does not mean being wasteful. You don't install documents in production. Be sane. Document only what's needed for regulatory, GAAP or other purposes. If you need to document in case someone leaves, you don't have collective code ownership and aren't pairing properly.

We treat both external documentation and internal documentation as a deliverable in a sprint. Working on documentation iteratively has the same benefits as working on code the same way.

What Agile is against is large up front specifications, not documentation per se.