If you have to communicate anything important — and in the workplace, a lot of the communication that occurs is important — you should always default to writing it down and sending the writeup to the the recipient by email or a chat service. Ideally, in fact, you would go one step further, and write an actual paper or wiki entry/paragraph with the information, and send the recipient a link to that — then, you get some documentation for free, and the recipient doesn't have to worry about remembering or jotting down everything you said. But even if you don't go that route, chat logs of a discussion are still a more reliable record than human memory, and the can provide the basis for real documentation when you actually sit down to write it out.
And you need to write everything down at some point, ideally sooner rather than later. Any information that has been communicated solely by voice should be considered officially lost, or close to it at least: even if the people present for the chat remember everything, they will eventually leave the project — or, heaven forbid, get hit by a bus — and take their knowledge with them. A new developer, starting after the entire existing team has left, should be able to learn their way around the project without having to resort to code archeology and best guesses. Old text chat conversations on their own also don't suffice: information stored in that fashion is very difficult to search effectively, and it will be lost if your team ever moves to a different communications platform. A central wiki or set of docs is therefore a must.
Finally, written sources are more accessible to a wide range of people, from non-native speakers of your common language to people with hearing or audio processing disorders. Processing text is also usually not a problem for people who have trouble seeing or reading, as text-to-speech technology is quite advanced these days. Don't exclude large swathes of people by refusing to provide information in the format they need it to be in.
In summary: write things down!