doc: Add a `constraint` default screenshot

lib/wibox/container/constraint.lua

@@ -1,4 +1,6 @@
 ---------------------------------------------------------------------------
+--
+--@DOC_wibox_container_defaults_constraint_EXAMPLE@
 -- @author Lukáš Hrázký
 -- @copyright 2012 Lukáš Hrázký
 -- @release @AWESOME_VERSION@

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

@@ -0,0 +1,27 @@
+--DOC_HIDE_ALL
+local wibox = require("wibox")
+
+return {
+    text   = "Some long text",
+    align  = "center",
+    valign = "center",
+    widget = wibox.widget.textbox,
+},
+{
+    {
+        {
+            {
+                text   = "Some long text",
+                align  = "center",
+                valign = "center",
+                widget = wibox.widget.textbox,
+            },
+            strategy = "max",
+            height   = 8,
+            width    = 50,
+            widget   = wibox.container.constraint
+        },
+        layout = wibox.layout.fixed.horizontal,
+    },
+    layout = wibox.layout.fixed.vertical,
+}