Add remark about HttpRequestMessage reuse (dotnet#8224)

Author: MihaZupan

xml/System.Net.Http/HttpRequestMessage.xml

@@ -42,6 +42,8 @@
   
 ## Remarks  
  The <xref:System.Net.Http.HttpRequestMessage> class contains headers, the HTTP verb, and potentially data.  
+
+ An <xref:System.Net.Http.HttpRequestMessage> instance should not be modified and/or reused after being sent.
   
  ]]></format>
     </remarks>