While working on #36502 and then #18236 about the definition and docs of str.isspace(), I looked closely also at its neighbor str.isprintable().

It turned out that we have the definition of what makes a character "printable" documented in three places, giving two different definitions.
    
The definition in the comment on `_PyUnicode_IsPrintable` is inverted, so that's an easy small fix.
    
With that correction, the two definitions turn out to be equivalent -- but to confirm that, you have to go look up, or happen to know, that those are the only five "Other" categories and only three "Separator" categories in the Unicode character database.  That makes it hard for the reader to tell whether they really are the same, or if there's some subtle difference in the intended semantics.

I've taken a crack at writing some improved docs text for a single definition, borrowing ideas from the C comment as well as the existing docs text; and then pointing there from the other places we'd had definitions. PR coming shortly.