Issue #3276619 by Matroskeen: Document XML parsers and explain the difference between them

src/Plugin/migrate_plus/data_parser/SimpleXml.php

@@ -10,6 +10,11 @@ use Drupal\migrate_plus\DataParserPluginBase;
 /**
  * Obtain XML data for migration using the SimpleXML API.
  *
+ * SimpleXML parses the whole file into memory, which allows using XPath
+ * expression in the item selector. For large XML sources it results in
+ * consuming lots of memory, which can be undesirable. If you run into memory
+ * issues, then consider using the 'xml' data parser.
+ *
  * @DataParser(
  *   id = "simple_xml",
  *   title = @Translation("Simple XML")

src/Plugin/migrate_plus/data_parser/Xml.php

@@ -10,6 +10,11 @@ use Drupal\migrate_plus\DataParserPluginBase;
 /**
  * Obtain XML data for migration using the XMLReader pull parser.
  *
+ * XMLReader reader performs incremental parsing of an XML file. This allows
+ * parsing very large XML sources (e.g. 200MB WordPress dumps), which reduces
+ * the memory usage and increases the performance. The disadvantage is that it's
+ * not possible to use XPath search across the entire source.
+ *
  * @DataParser(
  *   id = "xml",
  *   title = @Translation("XML")