ssb: Tweak idle scheduling even more, still. Fixes -t=bench.

src/ssb.h

@@ -807,7 +807,7 @@ typedef void(tf_ssb_scheduled_callback_t)(tf_ssb_connection_t* connection, bool
 /**
 ** Schedule work to be run when the connection is next idle.
 ** @param connection The owning connection.
-** @param key A key identifying the work.  If work by the same key already exists, it will be replaced.
+** @param key A key identifying the work.  If work by the same key already exists, the new request will be discarded.
 ** @param callback The callback to call.
 ** @param user_data User data to pass to the callback.
 */