The first rule of design club is … designing is not about writing documents.
The second rule of design club is … designing is not about writing documents.
If you are trying to design something at the same time as you are writing a document then it is fairly obvious that you can not be concentrating fully on either thing. Therefore, use whatever tools allow you to be creative during the design phase, but make sure you capture that information (in whatever format is appropriate) for the next phase of design.
Contrary to what seems to happen, design is iterative, and you need to make sure to that refine the design, the repository for that design work is flexible. People often talk about the importance of flexibility in code, but the same applies to all aspects of Software Engineering.