Document MultipartContent zero-part serialization behavior

Co-authored-by: MihaZupan <[email protected]>

Author: Copilot

xml/System.Net.Http/MultipartContent.xml

@@ -52,7 +52,14 @@
   </Attributes>
   <Docs>
     <summary>Provides a collection of <see cref="T:System.Net.Http.HttpContent" /> objects that get serialized using the multipart/* content type specification.</summary>
-    <remarks>To be added.</remarks>
+    <remarks>
+      <format type="text/markdown"><![CDATA[  
+  
+## Remarks  
+According to RFC 1341 (MIME) 7.2 and its successors, multipart messages must contain one or more body parts. When a <xref:System.Net.Http.MultipartContent> instance with zero parts is serialized, it's written as a single dummy empty part with no content headers or message body. This ensures compliance with the RFC requirement while maintaining backward compatibility. However, other HTTP clients might not interpret this message as a multipart content with zero parts, so it's recommended to add at least one part before serializing.  
+  
+ ]]></format>
+    </remarks>
   </Docs>
   <Members>
     <MemberGroup MemberName=".ctor">