KNOWN_BUGS: TLS session cache doesn't work with TFO

[skip ci]
Closes #4301

docs/KNOWN_BUGS

@@ -30,6 +30,7 @@ problems may have been fixed or changed somewhat since this was written!
  2.6 CURL_GLOBAL_SSL
  2.7 Client cert (MTLS) issues with Schannel
  2.8 Schannel disable CURLOPT_SSL_VERIFYPEER and verify hostname
+ 2.9 TLS session cache doesn't work with TFO
 
  3. Email protocols
  3.1 IMAP SEARCH ALL truncated response
@@ -256,6 +257,10 @@ problems may have been fixed or changed somewhat since this was written!
 
  https://github.com/curl/curl/issues/3284
 
+2.9 TLS session cache doesn't work with TFO
+
+ See https://github.com/curl/curl/issues/4301
+
 3. Email protocols
 
 3.1 IMAP SEARCH ALL truncated response