CACHE_BACKEND argument name doesn't match code

The code blocks where cull_frequency actually does its magic show that it is actually doing a frequency calculations:

doomed = [os.path.join(self._dir, k) for (i, k) in enumerate(filelist) if i % self._cull_frequency == 0]

and

doomed = [k for (i, k) in enumerate(self._cache) if i % self._cull_frequency == 0]

The default cull_frequency is 3, so the above code would flag every 3rd item as doomed (1/f or 1/3). I also have no skin in the game as I have not made use of this particular parameter, but I do think the code name, cull_frequency, is the more accurate description.

patch that does the opposite of ticket and replaces cull_percentage with cull_frequency

(In [10371]) Fixed a whole bunch of small docs typos, errors, and ommissions.

Fixes #8358, #8396, #8724, #9043, #9128, #9247, #9267, #9267, #9375, #9409, #9414, #9416, #9446, #9454, #9464, #9503, #9518, #9533, #9657, #9658, #9683, #9733, #9771, #9835, #9836, #9837, #9897, #9906, #9912, #9945, #9986, #9992, #10055, #10084, #10091, #10145, #10245, #10257, #10309, #10358, #10359, #10424, #10426, #10508, #10531, #10551, #10635, #10637, #10656, #10658, #10690, #10699, #19528.

Thanks to all the respective authors of those tickets.