https://www.naturaldocs.org/documentation/#File:Engine/Resou...
It's AGLPv3, which is usually OK for development tools because you aren't modifying/shipping them to customers - only the output/artifacts (a la GCC). But NaturalDocs will inject AGPLv3 artifacts (JS, CSS, etc.) into your doc output:
Portions of Natural Docs are automatically included with generated documentation in order to
help it function, such as JavaScript, CSS, and image files. These included files retain the
copyright and licensing terms of Natural Docs, but those terms do not apply to the remainder
of the generated documentation. All other generated documentation files remain under the
copyright and distribution terms decided by its author.
So if you're shipping docs generated by NaturalDocs to customers then you are shipping AGPLv3 and need to follow the license requirements.