Common sense tells me that these would be in the public domain with the intention that I can copy them as a starting point when creating my own Unity apps (otherwise what is the point in publishing them), however, without any other statement to the contrary, Section 1 of the Unity Website & Communities Additional Terms appears to suggest that we can’t use any content from the website commercially.
It does not appear to distinguish between copying the manual, creating derivative documentation, articles, or tutorials based on the Unity documentation, and using the code in our own Unity projects.