It happens regularly that a user complains about something but does not have a question.
When voting to close, would it make sense to have the option "not a question", or otherwise at least add this reason to the description of the "Needs details or clarity" option?
Per site, we have limited freedom in what we can modify in the close dialog. Specifically, we can only modify the community-specific reasons, and we cannot add further options (see note below about the details).
I suggest you to use the option "Needs details or clarity", as imperfect as it is, because the poster needs anyway to clarify what is the question. Note that this option is global for the whole network, and the likelihood of such a change—given the backlog and the priorities of the Stack Exchange staff—is probably tiny.
Note: Specifically, with the exception of Stack Overflow, standard sites have a limit of three community-specific close reasons, and we have already reached this limit. If there is a really strong case, and consensus about its need, a site can ask to have a fourth community-specific reason, but it's generally discouraged. About your suggestion, I can hardly see making a strong case around it.
