doc: Add a `mirror` default screenshot

lib/wibox/container/mirror.lua

@@ -1,4 +1,6 @@
 ---------------------------------------------------------------------------
+--
+--@DOC_wibox_container_defaults_mirror_EXAMPLE@
 -- @author dodo
 -- @copyright 2012 dodo
 -- @release @AWESOME_VERSION@

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

@@ -0,0 +1,19 @@
+--DOC_HIDE_ALL
+local wibox     = require("wibox")
+
+return {
+    text   = "Before",
+    align  = "center",
+    valign = "center",
+    widget = wibox.widget.textbox,
+},
+{
+    {
+        text   = "After",
+        align  = "center",
+        valign = "center",
+        widget = wibox.widget.textbox,
+    },
+    reflection = {horizontal = true},
+    widget     = wibox.container.mirror
+}