Most products have some sort of documentation or user manual. Is it appropriate to include in product requirements specifications about the content of such documentation? Is there such a thing as documentation requirements?
You certainly can specify documentation requirements. Keep in mind, however, that when you do so, you may not be paying sufficient attention to what really matters to the user. (And there's some question as to whether you properly can even call such specifications "requirements".)
This issue is almost identical to whether you should document support requirements separately, or whether support is part of your product.
Ideally, a product would have no documentation whatsoever. Documentation only benefits the user insofar as it decreases the learning curve and amount of effort it takes to use the product on an ongoing basis.
A holistic view of the product treats documentation, training, and support as part of the product. When you treat the product holisitically, the requirements constrain the whole package. Thus constraints on ease of use (e.g. the amount of time it takes a user with a given skill set to accomplish functional goals) encompass the user's experience with the product, documentation, training, and support.
Holistic requirements are almost essential if you really want to satisfy the user, because they capture what really matters to the user. The user doesn't care about documentation, training, or support, they just want to learn how to use the product as quickly as possible, and for it to be easy to use thereafter.
If you specify requirements on the documentation for a product, do so only after you've captured the holistic requirements.