Warn users about idleTimeout in Rocket

Summary: Idle timeout is known not to work with Rocket connections. Warn users about this.

Reviewed By: avalonalex

Differential Revision: D60602123

fbshipit-source-id: c53accb2395d8858b41b97e688970cc0c7a1c100

thrift/doc/languages/cpp/cpp2.md

@@ -62,6 +62,10 @@ Useful (but not complete set of) options that can be set on the ThriftServer:
 
 * `setIdleTimeout(std::chrono::milliseconds)` - milliseconds before we close idle connections.
 
+:::caution
+Idle timeout will not work for Rocket connections unless the `rocket_set_idle_connection_timeout` Thrift Flag is set to `true`.
+:::
+
 * `setTaskExpireTime(std::chrono::milliseconds)` - milliseconds before
   we timeout any individual request. This can also be set on a
   per-function bases by overriding the appropriate generated code

thrift/lib/cpp2/server/ThriftServer.h

@@ -1330,6 +1330,9 @@ class ThriftServer : public apache::thrift::concurrency::Runnable,
   /** Set maximum number of milliseconds we'll wait for data (0 = infinity).
    *  Note: existing connections are unaffected by this call.
    *
+   * WARNING: Idle timeout will not work for Rocket connections unless the
+   * rocket_set_idle_connection_timeout Thrift Flag is set to true.
+   *
    *  @param timeout number of milliseconds, or 0 to disable timeouts.
    */
   void setIdleTimeout(