Issue #2994319 by Ramya Balasubramanian, mohrerao, virajrajankar,...

/**

 * Provides an entity autocomplete form element.

 *

 * The #default_value accepted by this element is either an entity object or an

 * array of entity objects.

 * The autocomplete form element allows users to select one or multiple

 * entities, which can come from all or specific bundles of an entity type.

 *

 * Properties:

 * - #target_type: (required) The ID of the target entity type.

 * - #tags: (optional) TRUE if the element allows multiple selection. Defaults

 *   to FALSE.

 * - #default_value: (optional) The default entity or an array of default

 *   entities, depending on the value of #tags.

 * - #selection_handler: (optional) The plugin ID of the entity reference

 *   selection handler (a plugin of type EntityReferenceSelection). The default

 *   value is the lowest-weighted plugin that is compatible with #target_type.

 * - #selection_settings: (optional) An array of settings for the selection

 *   handler. Settings for the default selection handler

 *   \Drupal\Core\Entity\Plugin\EntityReferenceSelection\DefaultSelection are:

 *   - target_bundles: Array of bundles to allow (omit to allow all bundles).

 *   - sort: Array with 'field' and 'direction' keys, determining how results

 *     will be sorted. Defaults to unsorted.

 * - #autocreate: (optional) Array of settings used to auto-create entities

 *   that do not exist (omit to not auto-create entities). Elements:

 *   - bundle: (required) Bundle to use for auto-created entities.

 *   - uid: User ID to use as the author of auto-created entities. Defaults to

 *     the current user.

 * - #process_default_value: (optional) Set to FALSE if the #default_value

 *   property is processed and access checked elsewhere (such as by a Field API

 *   widget). Defaults to TRUE.

 * - #validate_reference: (optional) Set to FALSE if validation of the selected

 *   entities is performed elsewhere. Defaults to TRUE.

 *

 * Usage example:

 * @code

 * $form['my_element'] = [

 *  '#type' => 'entity_autocomplete',

 *  '#target_type' => 'node',

 *  '#tags' => TRUE,

 *  '#default_value' => $node,

 *  '#selection_handler' => 'default',

 *  '#selection_settings' => [

 *    'target_bundles' => ['article', 'page'],

 *   ],

 *  '#autocreate' => [

 *    'bundle' => 'article',

 *    'uid' => <a valid user ID>,

 *   ],

 * ];

 * @endcode

 *

 * @see \Drupal\Core\Entity\Plugin\EntityReferenceSelection\DefaultSelection

 *

 * @FormElement("entity_autocomplete")

 */