That is, are only words able to aptly describe the designs and nuances of a game? Does the lack of any visual conceptualization immediately nullifies the document's accuracy/effectiveness to be useful, despite how skillful a writer can be and what game type one is designing?
In part, it is meant to be somewhat broad as to be more inclusive.
I think it's pretty vital for the person that has come up with the idea to be able to present it visually. There is no other way for anyone else to fully appreciate what that person is imagining or seeing other than through drawing their ideas.
If this should be included in the design document is beyond me as i have never written or read one.