diff --git a/core/modules/migrate/src/Plugin/migrate/process/Iterator.php b/core/modules/migrate/src/Plugin/migrate/process/Iterator.php
index d30b2f5..adfd367 100644
--- a/core/modules/migrate/src/Plugin/migrate/process/Iterator.php
+++ b/core/modules/migrate/src/Plugin/migrate/process/Iterator.php
@@ -7,40 +7,38 @@
 use Drupal\migrate\Row;
 
 /**
- * Iterates and processes an array.
+ * Iterates and processes an associative array.
  *
- * The iterator process plugin allows processing of a list of associative arrays
- * Often source data will include multiple-value properties as associative arrays. 
- * For example, for filter formats on Drupal 6, the filters property contains a
- * list of filters belonging to that format, identified by a numeric delta
- * (a delta of 2 indicates the URL filter, a delta of 1 indicates automatic
- * breaking, etc.):
+ * The iterator process plugin allows sub-processing of a list of associative
+ * arrays. Often source data will include multiple-value properties as
+ * associative arrays. For example, for file uploads on Drupal 6, the upload
+ * property contains a list of sub-properties.
  *
  * Available configuration keys:
  *   - process: the plugin(s) that will process each element of the source.
+ *   - key: runs the process pipeline for the defined key.
  *
  * Examples:
  *
  * @code
  * source: Array
  * (
- *   [format] => 1
- *   [name] => Filtered HTML
- * ...
- *   [filters] => Array
+ *   [upload] => Array
  *     (
  *       [0] => Array
  *         (
- *           [module] => filter
- *           [delta] => 2
- *           [weight] => 0
+ *           [fid] => 1
+ *           [list] => 0
+ *           [description] => "File number 1"
  *         )
  *       [1] => Array
  *         (
- *           [module] => filter
- *           [delta] => 0
- *           [weight] => 1
+ *           [fid] => 2
+ *           [list] => 1
+ *           [description] => "File number 2"
  *         )
+ *     )
+ * )
  * ...
  * @endcode
  *
@@ -49,70 +47,40 @@
  *
  * @code
  * process:
- *   filters:
+ *   upload:
  *     plugin: iterator
- *     source: filters
+ *     source: upload
  *     process:
- *       id:
- *         plugin: static_map
- *         source:
- *           - module
- *           - delta
- *         map:
- *           filter:
- *             - filter_html_escape
- *             - filter_autop
- *             - filter_url
- *             - filter_htmlcorrector
- *             - filter_html_escape
- *           php:
- *              php_code
+ *       target_id:
+ *         plugin: migration_lookup
+ *         migration: d6_file
+ *         source: fid
+ *       display: list
+ *       description: description
  * @endcode
  *
- * So, plugin: iterator and source: filters means we take each array
- * element in the source filters field, and apply the static_map
- * plugin to it. We take the first ([0]) array:
+ * In this case, each item in the upload array will be processed bye the
+ * iterator process plugin. The target_id will be found by looking up the
+ * destination value from a previous migration. The display and description
+ * fields will simply be mapped.
  *
- * @code
- * Array
- * (
- *     [module] => filter
- *     [delta] => 2
- *     [weight] => 0
- * )
- * @endcode
- *
- * The map source keys are module and delta, so in the map key, since the
- * [module] value is filter, we index the array using the delta value of 2 and
- * arrive at filter_url as our value - this will be the first value in the array
- * assigned to the destination filters. Similarly, we will take the 0 value for
- * the weight in the second array to generate a second value of filter_html_escape.
- *
- * There's one more problem that needs to be solved: often the key of the arrays
- * need to be changed. In this case, the new id property needs to be the key.
- * It'd be possible to just add key: "@id" to the iterator process but that
- * would require special casing the key property and somehow escape it when
- * there is actually a destination called key. Instead, it is moved out of the
- * process and key is in the configuration of iterator itself:
+ * The mapping values passed to iterator are special. Numeric values may not be
+ * used.
  *
  * @code
- * process:
- *   filters:
- *     plugin: iterator
- *     source: filters
- *     key: "@id"
- *     process:
- *       id:
- *         plugin: static_map
+ *   field_relationship:
+       -
+         plugin: migration_lookup
+ *       migration:
+ *         - migration_page
+ *         - migration_blog
+ *       source: nid
+ *     -
+ *       plugin: iterator
+ *       process:
+ *         target_id: '0'
  * @endcode
- *
- * The process pipeline for key is special in two ways: first, it runs after
- * the normal process is done. Because of this, key: "@key" is also valid.
- * Second, the pipeline does not start with NULL but the value of the current key.
- * This way, the value of key can come from the other properties or the current
- * key. For the latter, omit the source from the first plugin in the pipeline
- * for key.
- *
+ * In the migration above, the value mapped to target_id is intentionally '0'.
  *
  * @see \Drupal\migrate\Plugin\MigrateProcessInterface
  *