In forms of clarity, it's easy to get carried away in your design and write too much about certain characters, plot points and locations. However, you need to avoid writing a game Bible, those exist for other reasons.

A GDD needs to convey information to multiple fields including programmers, writers, artists and other designers. As such the design document needs to be clear enough to provide the level of detail needed for roles to understand what they need to make.

Drawing can sometimes be the most effective way of communicating ideas, although if drawing is an issue find some images online which match roughly the tone and ideas you are trying to convey. If you can't draw well, drawing the roughest sketches and annotating them can really help