Mark MESOP_CONCURRENT_UPDATES_ENABLED as deprecated and update WebSoc…

…kets docs (#1223)

docs/api/config.md

@@ -241,11 +241,11 @@ If you want to show unredacted errors, including in prod mode, set this to `true
 
 These configuration values are experimental and are subject to breaking change, including removal in future releases.
 
-### MESOP_CONCURRENT_UPDATES_ENABLED
+### MESOP_CONCURRENT_UPDATES_ENABLED (DEPRECATED)
 
-!!! warning "Experimental feature"
+!!! danger "Deprecated experimental feature"
 
-      This is an experimental feature and is subject to breaking change. There are many bugs and edge cases to this feature.
+      This experimental feature is deprecated and will be removed in an upcoming release. Use `MESOP_WEBSOCKETS_ENABLED` instead.
 
 Allows concurrent updates to state in the same session. If this is not updated, then updates are queued and processed sequentially.
 
@@ -257,7 +257,14 @@ By default, this is not enabled. You can enable this by setting it to `true`.
 
     This is an experimental feature and is subject to breaking change. Please follow [https://github.com/google/mesop/issues/1028](https://github.com/google/mesop/issues/1028) for updates.
 
-This uses WebSockets instead of HTTP Server-Sent Events (SSE) as the transport protocol for UI updates. If you set this environment variable to `true`, then [`MESOP_CONCURRENT_UPDATES_ENABLED`](#MESOP_CONCURRENT_UPDATES_ENABLED) will automatically be enabled as well.
+This uses WebSockets instead of HTTP Server-Sent Events (SSE) as the transport protocol for UI updates. Using WebSockets also changes the Mesop server architecture to be stateful (e.g. meaning a server restart or routing to a different server instance means the user will lose state). If you enable this feature, then you should ensure the following:
+
+- Either a single server instance is used *or* you route a particular user to the same server for a given session (i.e. [session affinity](https://cloud.google.com/run/docs/configuring/session-affinity)).
+- Either ensure user state is *not* critical (e.g. it's OK if it's cleared out due to server restart, which could happen at any time on typical cloud infrastructure) or is persisted somewhere (e.g. written to a database).
+
+Enabling this also allows user events to be handled concurrently, which is particularly useful supporting interactivity for long-running event handlers.
+
+By default, this is not enabled. You can enable this by setting it to `true`.
 
 ### MESOP_APP_BASE_PATH