The second value listed in the Agile Manifesto is:
Woking software over comprehensive documentation
Notice the use of the word "comprehensive" above. The authors are not saying don't do any documentation at all. They are not even saying not to do comprehensive documentation. I think they are saying that where you have a choice of activities to perform that working software has more value than a fully documented product.
In the real world this means getting the product right takes precedence over completing a fully detailed manual. In many of the Scrum teams I have worked in documentation is almost non-existent. These teams have leaned too heavily on the side of "working software" and neglected documentation. I understand how easy this can be, but I caution against this.
How to include at least some documentation?
As part of your definition of done, you can add that the story/task must have minimal documentation. Or you could add it as a requirement to tasks if you decide that it's relevant.
But don't do it for the sake of doing it - that is the kind of wastefulness that Agile tries to avoid. My advice would be to document where there is value in doing so. Your product owner should be able to provide good guidance on the appropriate value of documentation.