Issue #3500774 by kul.pratap, benjifisher: Fix errors in SourcePluginBase doc block

 *     \Drupal\migrate\Plugin\MigrateIdMapInterface::STATUS_NEEDS_UPDATE.

 * - The row needs an update.

 *   - Rows can be marked by custom or contrib modules using the

 *     \Drupal\migrate\Plugin\MigrateIdMapInterface::prepareUpdate() os

 *     \Drupal\migrate\Plugin\MigrateIdMapInterface::prepareUpdate() or

 *     \Drupal\migrate\Plugin\MigrateIdMapInterface::setUpdate()

 *     methods.

 * - The row is above the high-water mark.

 * source:

 *   plugin: some_source_plugin_name

 *   constants:

 *     - foo: bar

 *     foo: bar

 * process:

 *   baz: constants/bar

 *   baz: constants/foo

 * @endcode

 *

 * In this example, the constant 'foo' is defined with a value of 'bar'. It is