Clarify CookieContainer thread-safety

Author: antonfirsov

xml/System.Net/CookieContainer.xml

@@ -79,6 +79,20 @@
 
  ]]></format>
     </remarks>
+    <threadsafe>
+      <format type="text/markdown"><![CDATA[
+
+## Thread Safety
+ 
+  The methods for adding and retrieving <xref:System.Net.Cookie> instances to/from a <xref:System.Net.CookieContainer> are thread-safe and may be used concurrently from multiple threads.
+
+  > [!NOTE]
+  > Regardless of thread-safety, unanticipated sharing of <xref:System.Net.CookieContainer> may lead to issues when working with <xref:System.Net.Http.HttpClient> and <xref:System.Net.Http.HttpClientHandler> or <xref:System.Net.Http.SocketsHttpHandler>.
+  > One such problem is isolation: parts of the application that not intended to share cookies may inadvertently do so.
+  > It is not recommended to use cookies together with <xref:System.Net.Http.IHttpClientFactory>.
+
+ ]]></format>
+    </threadsafe>
     <altmember cref="T:System.Net.Cookie" />
     <altmember cref="T:System.Net.CookieCollection" />
     <altmember cref="T:System.Net.CookieContainer" />