The purpose of the design document is to be the guide for someone who needs to understand the code to maintain it, extend it, or re-use parts of it. While “all of the information” is in the code, the concepts that underlie it are lost in the details. Consider this document a tutorial for our code. While it is true that the reader of the document will be familiar with basic programming and object-oriented concepts, that is all that we can assume.
In the first two sections, you are more apt to find that graphical representation helps in the understanding, but in the later sections, you will probably find that text will work fine.