[$] Readahead: the documentation I wanted to read

The readahead code in the Linux kernel is nominally responsible forreading data that has not yet been explicitly requested from storage,with the idea that it might be needed soon. The code is stable, functional, widelyused, and uncontroversial, so it is reasonable to expect the code to be ofhigh quality, and largely this is true. Recently, I found the need todocument this code, which naturally shone a rather different light onit. This work revealed minor problems with functionality and significantproblems with naming.