If you make an exit such as a door, and the user then says they want to go in that direction with the door, how do you write a description of that door before they walk through it? Also, can you ask them a question about whether they are sure they want to walk through that door?

You can either enter the description in the section marked "Before exiting the room" in the room they're currently in, or in the section labelled "Before entering the room" of the room they're moving in to.