Is there a Blender icon reference for the Outliner editor? Today I saw a ghost icon and broken paper icon in Outliner. My guess is they mean proxy object and missing file, but haven't found any documentation to confirm.

Is there a reference for the various icons used in the Outliner? The best thing I've found so far is the Data-Block Types documentation which includes this icon reference:

However, this does not include the ghost or broken paper icons.
Use the Icon Viewer add-on and outliner_draw.c file.

Enable the Icon Viewer add-on.
In Icon Viewer, click on the icon you're trying to lookup. 
In an Info editor, note the name of the icon. This may be descriptive enough on it's own, but if not, proceed to the next step. In this example, it is GHOST_ENABLED. 
Go to outliner_draw.c and look for the same text, GHOST_ENABLED in this example. You should find the type of object the icon represents in the case statement right above the icon name. In this example, you can see it is a proxy. 

Using the same process for the broken paper icon shows it is LIBRARY_DATA_BROKEN.