diff --git a/core/modules/migrate/src/Plugin/migrate/process/Iterator.php b/core/modules/migrate/src/Plugin/migrate/process/Iterator.php
index 2a41c4a..5ae94f6 100644
--- a/core/modules/migrate/src/Plugin/migrate/process/Iterator.php
+++ b/core/modules/migrate/src/Plugin/migrate/process/Iterator.php
@@ -7,9 +7,114 @@
 use Drupal\migrate\Row;
 
 /**
- * This plugin iterates and processes an array.
+ * Iterates and processes an array.
  *
- * @link https://www.drupal.org/node/2135345 Online handbook documentation for iterator process plugin @endlink
+ * 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.):
+ *
+ * Available configuration keys:
+ *   - process:
+ *
+ * Examples:
+ *
+ * @code
+ * source: Array
+ * (
+ *   [format] => 1
+ *   [name] => Filtered HTML
+ * ...
+ *   [filters] => Array
+ *     (
+ *       [0] => Array
+ *         (
+ *           [module] => filter
+ *           [delta] => 2
+ *           [weight] => 0
+ *         )
+ *       [1] => Array
+ *         (
+ *           [module] => filter
+ *           [delta] => 0
+ *           [weight] => 1
+ *         )
+ * ...
+ * @endcode
+ *
+ * The iterator process plugin will take these arrays one at a time and run its
+ * own process over each one:
+ *
+ * @code
+ * process:
+ *   filters:
+ *     plugin: iterator
+ *     source: filters
+ *     process:
+ *       id:
+ *         plugin: static_map
+ *         source:
+ *           - module
+ *           - delta
+ *         map:
+ *     filter:
+ *       0: filter_html_escape
+ *       1: filter_autop
+ *       2: filter_url
+ *       3: filter_htmlcorrector
+ *       4: filter_html_escape
+ *     php:
+ *       0: php_code
+ * @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:
+ *
+ * @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:
+ *
+ * @code
+ * process:
+ *   filters:
+ *     plugin: iterator
+ *     source: filters
+ *     key: "@id"
+ *     process:
+ *       id:
+ *         plugin: static_map
+ * @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.
+ *
+ *
+ * @see \Drupal\migrate\Plugin\MigrateProcessInterface
  *
  * @MigrateProcessPlugin(
  *   id = "iterator",