Unaware users may be confused by why the tray module is not shown in the
default config and if they add it, they may be confused why only one of
their bars shows the module.
Substitutions in the caption are only supported in version 4 of
sphinx.
However, RTD uses sphinx 1.8.6
Just letting it append the value seems to work in both 1.8.6 and newer
versions.
