From what I can tell, it looks like you can put it pretty much anywhere.
For example, I unzipped it to my desktop, so I had a p5-reference directory on my desktop.
Then I opened that directory and I saw there was an index.html file, so I opened that in my web browser.
That opened my web browser to this local url: file:///C:/Users/kevin/Desktop/p5-reference/index.html
Which showed an offline version of the reference:
What I think might be confusing you is that I’m not sure if there’s any integration between the Processing editor and the p5.js offline reference. I would probably open the reference in my browser like above, rather than going through the Processing editor.
Does that work?