There are (at least) two things you can mean by UserManual.
The distinction between the UserManual and other documents that might be produced for a software project is that the customer is expected to be the primary audience. I wouldn't expect to see a UML scenario included in a UserManual, for instance. A language reference might be part of a UserManual for a compiler, using the second sense.
Some software systems might not have much of a UserManual. Small programs with intuitive interface, like a text editor with nothing fancy in the way of features might need nothing more than a README and a copyright notice. Large systems with many executables and lots of configuration options, say an IntegratedDevelopmentEnvironment, might require so many documents as to require several CDs to hold it.
-- SkipSailors
It has been suggested that the UserManualIsAnAntiPattern, but it seems that most people on this wiki think that a MostApplicationsNeedaUserManual. [DeleteWhenCooked: These two pages are being carved out of UserManualIsAntiPattern.]