The distinguishing feature of an "agile" techniques is it's approach to risk. We have the humility to admit that development is primarily a learning process, and that risk originates from factors that are initially unknown.
Agile techniques seek to reduce development risk by producing a minimum viable product as early as possible, by postponing critical decisions until sufficient information is available, and by learning-by-doing. Storage and dissemination of lessons-learned is a critical part of any non-trivial agile effort.
Documentation is therefore critically important, but because documentation may change significantly and rapidly (as the team's understanding of the problem evolves) the form that the documentation takes must be very different in an agile development organization from a classical development organization.
I cannot stress this enough: Development Automation is THE key enabling set of technologies that enables us to take an agile approach. Documentation must be written in a formal manner, accessible to automation and able to be changed rapidly with minimal effort.
It also must be parsimonious and accessible enough to be disseminated rapidly. If automation is limited, then documentation cannot be lengthy. If automation is sophisticated, then more documentation can exist. Ultimately, the quantity of documentation is limited by the ability of team members to absorb the information rather than the speed with which it can be re-written.
Well written, readable source documents meet these criteria.