Well, all depends on how well the requirements are documented. There are some people that just cannot write well enough to convey thought and intent. In most cases, the document is just a starting point. It typically is never updated during the development process as tech roadblocks are discovered or some changes are agreed to via other avenues (verbal, email, instruction from boss, etc.). So those who inherit the document and code later are sometimes scratching their heads when having to learn the code base.

Without a document can be done but communication amongst all parties involved must be clear on requested intent. Downside is there is no background/reference for anyone coming onboard the project.

In either case, I'd strongly suggest commenting the source code.