Document XML parser usage against security false positives

Prior to this commit, our XML parser usage would be already haredened
against XXE (XML External Entities) attacks. Still, we recently received
several invalid security reports claiming that our setup should be
hardened.

This commit documents a few usages of XML parsers to add some more
context and hopefully prevent future invalid reports.

Closes gh-33713

spring-oxm/src/main/java/org/springframework/oxm/jaxb/Jaxb2Marshaller.java

@@ -603,6 +603,9 @@ public class Jaxb2Marshaller implements MimeMarshaller, MimeUnmarshaller, Generi
 		Assert.hasLength(schemaLanguage, "No schema language provided");
 		Source[] schemaSources = new Source[resources.length];
 
+		// This parser is used to read the schema resources provided by the application.
+		// The parser used for reading the source is protected against XXE attacks.
+		// See "processSource(Source source)".
 		SAXParserFactory saxParserFactory = this.schemaParserFactory;
 		if (saxParserFactory == null) {
 			saxParserFactory = SAXParserFactory.newInstance();
@@ -907,6 +910,8 @@ public class Jaxb2Marshaller implements MimeMarshaller, MimeUnmarshaller, Generi
 		}
 
 		try {
+			// By default, Spring will prevent the processing of external entities.
+			// This is a mitigation against XXE attacks.
 			if (xmlReader == null) {
 				SAXParserFactory saxParserFactory = this.sourceParserFactory;
 				if (saxParserFactory == null) {