docs(pro): slow drain disconnection

docs/configuration.md

@@ -211,7 +211,7 @@ $ anycable-go \
 
 You can also monitor the current concurrency value via the `rpc_capacity_num` metrics.
 
-## Disconnect events settings
+## Disconnect settings
 
 AnyCable-Go notifies an RPC server about disconnected clients asynchronously with a rate limit. We do that to allow other RPC calls to have higher priority (because _live_ clients are usually more important) and to avoid load spikes during mass disconnects (i.e., when a server restarts).
 
@@ -239,6 +239,16 @@ Using `--disconnect_mode=always` is useful when you have some logic in the `Appl
 
 **NOTE:** AnyCable tries to make a Disconnect call for active sessions during the server shutdown. However, if the server is killed with `kill -9` or crashes, the disconnect queue is not flushed, and some disconnect events may be lost. If you experience higher queue sizes during deployments, consider increasing the shutdown timeout by tuning the `--shutdown_timeout` parameter.
 
+### Slow drain mode
+
+<p class="pro-badge-header"></p>
+
+AnyCable-Go PRO provides the **slow drain** mode for disconnecting clients during shutdown. When it is enabled, AnyCable do not try to disconnect all active clients as soon as a shutdown signal is received. Instead, spread the disconnects over the shutdown timeout period.
+
+Use this feature to reduce the load on AnyCable servers during deployments (i.e., to avoid the _thundering herd_ situation).
+
+**NOTE:** This feature only makes sense if you peform rolling updates, or blue-green deployement, or similar.
+
 ## GOMAXPROCS
 
 We use [automaxprocs][] to automatically set the number of OS threads to match Linux container CPU quota in a virtualized environment, not a number of _visible_ CPUs (which is usually much higher).