doc: Add a `background` default screenshot

lib/wibox/container/background.lua

@@ -1,6 +1,8 @@
 ---------------------------------------------------------------------------
 -- A container capable of changing the background color, foreground color
 -- widget shape.
+--
+--@DOC_wibox_container_defaults_background_EXAMPLE@
 -- @author Uli Schlachter
 -- @copyright 2010 Uli Schlachter
 -- @release @AWESOME_VERSION@

tests/examples/wibox/container/defaults/background.lua

@@ -0,0 +1,24 @@
+--DOC_HIDE_ALL
+local wibox     = require("wibox")
+local gears     = {shape = require("gears.shape")}
+local beautiful = require("beautiful")
+
+return {
+    text   = "Before",
+    align  = "center",
+    valign = "center",
+    widget = wibox.widget.textbox,
+},
+{
+    {
+        text   = "After",
+        align  = "center",
+        valign = "center",
+        widget = wibox.widget.textbox,
+    },
+    shape              = gears.shape.circle,
+    shape_border_width = 5,
+    shape_border_color = "#ff0000",
+    bg                 = beautiful.bg_highlight,
+    widget             = wibox.container.background
+}