These things are being called "hacks" in the tradition of the older _Unix Power Tools_ and _X Power Tools_ books, which first introduced the visual & writing style that the current run of Hacks books are now using.
While much of the material in those books was novel & creative, at the same time a lot of those hints were, as you say, available in the system documentation, such as the big printed Sun manual (you know, the one that was four feet long on a special metal rack). The original "Power Tools" books distilled that mix of new & old information down to one pragmatic, easily accessible, and nicely cross-referenced book.
I'm not sure I see what the harm is in calling material like this a "hack". *shrug*