Instruction manuals are typically written in a way that acknowledges that the video game is a fictional thing being created by the player's computer. However, some developers want the player's experience of the world to begin as early as possible, and to that end they create documents which explain the game's mechanics and features from a game character's perspective. Sometimes this takes the form of an instruction book from the world, other times a character's diary.
Another common feature of such manuals is the inclusion of handwritten notes from the game's characters. Crusader: No Remorse is a good example of this technique.