Maybe people are suppose to write-in their own observations and experiences with women??
No. It means there's nothing we know about women. In other words, we don't know anything about women. That's why the author can't even write anything down.
I think the author is referring to a woman's behaviors though. There are things we do know about women: women conceive and give birth, proliferating life of that kind; women eat foods and drink liquid just as men do. Um...that might be it.