systray: Enable properties and improve documentation

I spent way too much time trying to figure out why setting "visible" to
false did not work...

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/wibox/widget/systray.lua

@@ -161,7 +161,7 @@ end
 -- @function wibox.widget.systray
 
 local function new(revers)
-    local ret = wbase.make_widget()
+    local ret = wbase.make_widget(nil, nil, {enable_properties = true})
 
     gtable.crush(ret, systray, true)
 
@@ -191,6 +191,10 @@ function systray.mt:__call(...)
     return instance
 end
 
+--@DOC_widget_COMMON@
+
+--@DOC_object_COMMON@
+
 return setmetatable(systray, systray.mt)
 
 -- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80