Issue #2973713 by quietone, Adita, etecjdo, apmsooner, mikeryan, gnuschichten,...

Issue #2973713 by quietone, Adita, etecjdo, apmsooner, mikeryan, gnuschichten, tstoeckler: cache_key source plugin configuration not documented

core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php

@@ -17,6 +17,7 @@
  *
  * Available configuration keys:
  * - cache_counts: (optional) If set, cache the source count.
+ * - cache_key: (optional) Uniquely named cache key used for cache_counts.
  * - skip_count: (optional) If set, do not attempt to count the source.
  * - track_changes: (optional) If set, track changes to incoming data.
  * - high_water_property: (optional) It is an array of name & alias values