CSSTUDIO-2648 Add documentation.

core/ui/src/main/resources/phoebus_ui_preferences.properties

@@ -52,9 +52,12 @@ home_display=examples:/01_main.bob?app=display_runtime,Example Display
 #
 # The special entry "*" adds all remaining available toolbar entries.
 # An entry starting with "!" removes that item from the available entries.
-# The order of the initial buttons "Home, Top Resources, Layouts"
+# The order of the initial buttons "Home, Top Resources, Layouts, Add Layouts"
 # cannot be changed, but they can be suppressed by adding "!",
-# for example "Home, !Top Resources, !Layouts".
+# for example "Home, !Top Resources, !Layouts, !Add Layouts".
+#
+# The special entry "Add Layouts" is NOT added to the toolbar by default. It
+# can be added to the toolbar by specifying "Add Layouts".
 #
 # The strings in the list of entries MUST match what is returned from
 # ToolbarEntry#getId(). This allows for customization/localization of app name