Allow pacman's update_cache to be run by itself

The module was also cleaned up to conform with the more standard terms 'present' and 'absent' (although 'installed' and 'removed' still work), as well as rewording and making the documentation more consistent. I did remove trailing whitespace on a few lines, but I was touching most of the file for this update anyway.

`git diff --patience` makes this much easier to read, but *all changes are backward compatible*.