I just fell prey to this myself. The “show full post” button on my site decided to show the privacy policy from my site, and not the full post. So I had to disable it. I went to search admin settings for “show full post” but it is not included in the helper text so shows no result. I had to come back to this post to get the answer. Thanks, @simon!
So.. agreeing with @codinghorror that the help copy should be improved, at least to include the keywords “show full post”. For the rest I don’t fully understand how this functionality works and will leave it off on my site for the time being.