Apply suggestion from @MihaZupan

Author: MihaZupan

xml/System.Net.Http/MultipartContent.xml

@@ -56,7 +56,8 @@
       <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.
+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 empty part with no content headers or message body.
+If your server does not support such contents with zero parts, consider conditionally using a different or no `HttpContent`.
 
 ]]></format>
     </remarks>