Document MultipartContent zero-part serialization behavior (#12014)

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Co-authored-by: MihaZupan <25307628+MihaZupan@users.noreply.github.com>
Co-authored-by: Miha Zupan <mihazupan.zupan1@gmail.com>
Co-authored-by: Genevieve Warren <24882762+gewarren@users.noreply.github.com>

Author: 3 people

xml/System.Net.Http/MultipartContent.xml

@@ -52,7 +52,10 @@
   </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>
+Multipart messages must contain one or more body parts. When a <see cref="T: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 doesn't support such contents with zero parts, consider conditionally using a different <see cref="T:System.Net.Http.HttpContent" /> type or no <see cref="T:System.Net.Http.HttpContent" />.
+    </remarks>
   </Docs>
   <Members>
     <MemberGroup MemberName=".ctor">