Extend TFO docs (#11196)

Author: antonfirsov

xml/System.Net.Sockets/SocketOptionName.xml

@@ -741,8 +741,7 @@
       </ReturnValue>
       <MemberValue>15</MemberValue>
       <Docs>
-        <summary>Enables TCP Fast Open as defined in RFC-7413. The actual observed behavior depends on OS configuration and the state of the kernel TCP cookie cache.
- Enabling TFO can impact interoperability and cause connectivity issues.</summary>
+        <summary>Enables TCP Fast Open as defined in RFC-7413. The actual observed behavior depends on OS configuration and the state of the kernel TCP cookie cache. Using TFO is only recommended in controlled networking environments, meaning that each middlebox (for example, firewall, router, proxy) on the networking path between the client and server is known and can be configured to support TFO. Using TFO outside of controlled environments (such as the open internet) can lead to connectivity issues.</summary>
       </Docs>
     </Member>
     <Member MemberName="HeaderIncluded">