userguide: add section with synonym "screen" for "RandR output"

While the docs describe the feature, plenty of people ask for it on IRC.
Let’s see if this makes it easier to find.

docs/userguide

@@ -1524,6 +1524,11 @@ i3-msg 'rename workspace to "2: mail"
 bindsym $mod+r exec i3-input -F 'rename workspace to %s' -P 'New name: '
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+=== Moving workspaces to a different screen
+
+See <<move_to_outputs>> for how to move a container/workspace to a different
+RandR output.
+
 === Moving containers/workspaces to RandR outputs
 
 [[move_to_outputs]]