Issue #2848367 by mlncn, jp.stacey, kunalkursija: Render API overview example...

 * render array contained:

 * @code

 * $build['my_element'] = [

 *   '#attached' => ['placeholders' => ['@foo' => 'replacement']],

 *   '#markup' => ['Something about @foo'],

 *   '#markup' => 'Something about @foo',

 *   '#attached' => [

 *     'placeholders' => [

 *       '@foo' => ['#markup' => 'replacement'],

 *     ],

 * ];

 * @endcode

 * then #markup would end up containing 'Something about replacement'.

 *

 * Note that each placeholder value can itself be a render array, which will be

 * Note that each placeholder value *must* itself be a render array. It will be

 * rendered, and any cache tags generated during rendering will be added to the

 * cache tags for the markup.

 *