The Blender Manual is right. There is a viewer nodeviewer node under Output, but it's in the Composite Node Editor. I bet you are looking in the Material Node Editor.
To get one in the Material Node Editor, you have to activate the Node Wrangler add-onNode Wrangler add-on, then you can use Ctrl+Shift+LMB+LMB to create a temporary Viewer Nodetemporary Viewer Node.