Update description of viewer output in docs

doc/source/user_guide/viewer.txt

@@ -34,12 +34,16 @@ let's see an example of how a pre-defined plugin is added to the viewer:
    viewer += LineProfile(viewer)
    overlay, data = viewer.show()[0]
 
-The viewer's ``show`` method will return an overlay (of the same shape as the
-input image) and data (possibly ``None``) for each attached plugin. In this
-case, there is only one plugin, so we can directly name both. You can see the
-plugin class's ``output`` method to see what will be returned. Here,
-``overlay`` contains a drawing of the line (including its width), and ``data``
-contains the measured line profile.
+The viewer's ``show()`` method returns a list of tuples, one for each attached
+plugin. Each tuple contains two elements: an overlay of the same shape as the
+input image, and a data field (which may be ``None``). A plugin class documents
+its return value in its ``output`` method.
+
+In this example, only one plugin is attached, so the list returned by ``show``
+will have length 1. We extract that single tuple and bind its ``overlay`` and
+``data`` elements to individual variables. Here, ``overlay`` contains an image
+of the line drawn on the viewer, and ``data`` contains the 1-dimensional
+intensity profile along that line.
 
 At the moment, there are not many plugins pre-defined, but there is a really
 simple interface for creating your own plugin. First, let us create a plugin to
@@ -83,8 +87,8 @@ All that's left is to create an image viewer and add the plugin to that viewer.
    viewer += denoise_plugin
    denoised = viewer.show()[0][0]
 
-When we close the viewer, ``denoised`` will contain the filtered image for the
-last used setting of ``weight``.
+Here, we access only the overlay returned by the plugin, which contains the
+filtered image for the last used setting of ``weight``.
 
 .. image:: data/denoise_viewer_window.png
 .. image:: data/denoise_plugin_window.png