ext/libxml: Add a warning about the usage of LIBXML_PARSEHUGE (#5647)

Author: alexandre-daubois

reference/libxml/constants.xml

@@ -277,6 +277,13 @@
      limits like maximum depth of a document or the entity recursion, as well as limits of the
      size of text nodes.
     </simpara>
+    <caution>
+     <simpara>
+      Because this relaxes the hardcoded limits, it should only be used with trusted data.
+      Removing the depth limit on untrusted input can lead to excessive resource consumption,
+      such as a stack overflow while processing a deeply nested document.
+     </simpara>
+    </caution>
     <note>
      <para>
       Only available in Libxml &gt;= 2.7.0 (as of PHP &gt;= 5.3.2 and PHP &gt;= 5.2.12)