
There are some pretty good tool integrations with Wiki's that provide a various nice way of being able to update/edit story descriptions (either in the wiki, or the tool) and being able to organize+navigate a hierarchical "tree" of epics & stories as if it were an online document, as well as being able to generate (or print) a single document from the tree of stories.
My favorite way however is in the case where you already know there is a need for user-documentation (e.g., user-guides, online help, reference-manual, admin-guide, etc.). and to have these documents serve double-duty as system/requirements documentation that incremental evolve with each iteration. This may involve "pairing" those who detail the story (and the tests) with technical writer, as well as using practices like BDD and Specification by Example to generate some of the examples used in the user-docs.