Blender itself is released under the GNU General Public License. More info blender.org/about/license.
Except where otherwise noted, the content of the Blender Manual is available under a Creative Commons Attribution-ShareAlike 4.0 International License or any later version. Excluded from the CC-BY-SA are also the used logos, trademarks, icons, source code and Python scripts.
Please attribute the "Blender Documentation Team" and include a hyperlink (online) or URL (in print) to manual. For example:
The Blender 2.83 Manual by the Blender Documentation Team is licensed under a CC-BY-SA v4.0.
See Best practices for attribution for further explanation.
It means, that when contributing to the manual you do not hold exclusive copyright to your text. You are, of course, acknowledged and appreciated for your contribution. However, others can change and improve your text in order to keep the manual consistent and up to date.
If you have questions about the license, feel free to contact the Blender Foundation: foundation (at) blender (dot) org
Previous versions of the Blender Manual are made available under a Open Content License v2.26 -- v2.77.