diff -u b/core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php b/core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php
--- b/core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php
+++ b/core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php
@@ -16,9 +16,9 @@
  * The base class for source plugins.
  *
  * Available configuration keys:
- * - cache_counts: Whether this instance should cache the source count.
- * - skip_count: Whether this instance should not attempt to count the source.
- * - track_changes: Flags whether to track changes to incoming data.
+ * - cache_counts: (optional) If set it will cache the source count.
+ * - skip_count: (optional) If set it will not attempt to count the source.
+ * - track_changes: (optional) If set it will track changes to incoming data.
  * - high_water_property: Information on property used as the high-water mark.
  *
  * @see \Drupal\migrate\Plugin\MigratePluginManager