[RFC] Clarify some castings in portable format specifiers

@justinmk pointed out some cases where the introduced castings could be incorrect (see https://github.com/neovim/neovim/pull/574#discussion-diff-11974822).

Though, to my understanding, they are correct in their context, I do agree they can be confusing, and, as explained in the discussion, me myself don't like them that way and only did it so because the semi-automated process I used was easier that way.

In any case, I have reviewed them to eliminate any possible doubt about this.