Writing documentation up front is a good idea, but only if one treats it as the first iteration of the program with the expectation that the initial documentation may well be thrown out. The process of writing the documentation no doubt will help you organize the plan for writing the code. But there will also be things that the documentation does not capture or problems it does not foresee that emerge as the program is being written.

The danger is that either you fail to update the documentation to account for the changes in the system that emerge in development or you only update parts of the documentation as you go which causes the documentation to become inconsistent and unreliable.

A middle ground is write the documentation up front and then rewrite it after the system is done. The initial draft helps guide the design and the final version captures the full and complete essence of the finished program, which is nearly impossible to do up front.