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This feature has been removed in Office2013.
It isn't really a question of what appears on the taskbar, but rather the behavior of the windows that hold the documents. (Thanks to MS for mislabeling the option and creating the confusion!)
In Word XP,2003,2007, and2010, when the option "Windows in Taskbar" or "Show all windows in taskbar" is checked and you open multiple documents, each document gets its own set of menus or ribbons and behaves as a completely separate window. (The technical term is Single Document Interface or SDI.)
In those versions, when that option is unchecked and you open multiple documents, there is only one set of menus or ribbons. You can tile or cascade the documents within the main Word window. (The term for this is Multiple Document Interface or MDI.) The fact that there's only one icon on the taskbar is really a side effect, but it was easier to dumb down the option name than to explain the difference between SDI and MDI.
In Word2013, MDI no longer exists, and the option doesn't appear in the dialog.
The interesting part is that Microsoft dropped MDI from Word2000, and the outcry was so loud that they had to put it back in Word XP (a.k.a. Word2002).
