A document is considered to follow the agile principle if it is valuable, essential, and created or updated just-in-time. A documentation is created and maintained in an agile way, if all its documents follow this practice.
All publishing organizations define a style guide for their published information. Such a guide supports teams to write in a similar tone, making it easier for readers to digest the information.
In order to take personal notes on one's own work and to reflect upon what has to be done or has been done, keep a journal. The information in the journal should be shareable at least with all team members.
Define a basic structure for all artifacts of a given type. Readers will have an easier job on finding and learning about the information in your documentation.