Issue #3247040 by _shY, quietone, smustgrave, joachim: MigrateSkipRowException...

Issue #3247040 by _shY, quietone, smustgrave, joachim: MigrateSkipRowException should document that it is for use within migration process plugins

(cherry picked from commit e03d3ec5)

core/modules/migrate/src/MigrateSkipRowException.php

@@ -4,6 +4,10 @@
 
 /**
  * This exception is thrown when a row should be skipped.
+ *
+ * This exception should be used in Migrate process plugins. Throwing it in a
+ * source plugin may cause unexpected results in the count of rows processed.
+ * And throwing it in a destination plugin causes an error.
  */
 class MigrateSkipRowException extends \Exception {