Document data_table entity info key

Problem/Motivation

(Followup for #1763974: Convert entity type info into plugins)

The data_table key of the entity info array (previously data table) is not documented. It is used in the following places in core:

[lorentz:drupal | Tue 10:31:58] $ grep -r "data_table" * | grep -v ".patch"
core/modules/field/modules/field_sql_storage/field_sql_storage.install:    $data_table = _field_sql_storage_tablename($field);
core/modules/field/modules/field_sql_storage/field_sql_storage.install:    $table_info = array($data_table => $primary_key_data, $revision_table => $primary_key_revision);
core/modules/field/modules/field_sql_storage/lib/Drupal/field_sql_storage/Entity/Query.php:    if (isset($entity_info['data_table'])) {
core/modules/field/modules/field_sql_storage/lib/Drupal/field_sql_storage/Entity/Query.php:      $entity_tables[$entity_info['data_table']] = drupal_get_schema($entity_info['data_table']);
core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Plugin/Core/Entity/EntityTest.php: *   data_table = "entity_test_property_data",

Proposed resolution

Add documentation for the key in the docblock for the EntityManager.