Simplified multi-queue docs

Author: matt-aitken

docs/queue-concurrency.mdx

@@ -168,20 +168,14 @@ export const perUserQueue = queue({
   name: "per-user-queue",
   //each user runs at most 1 at a time...
   concurrencyLimit: 1,
-  //...and at most 10 users can be running at once
+  //...and at most 10 total runs across all users
   combinedConcurrencyLimit: 10,
 });
 ```
 
 The combined limit only applies to runs triggered with a `concurrencyKey`; runs without a key are governed by `concurrencyLimit` alone. On the Queues page in the dashboard, a queue with a combined limit shows it in brackets next to the per-key limit, e.g. `1 (10)`.
 
-<Note>
-  If you self-host, combined limits are enforced by default and can be disabled with
-  `RUN_ENGINE_TOTAL_CONCURRENCY_LIMITS_ENABLED=0`. When enforcement is disabled the limit is
-  still accepted, stored, and shown, but runs are not held back by it.
-</Note>
-
-## Holding slots in more than one queue (queue gates)
+## Using multiple queues at once
 
 Sometimes one limit isn't enough: each tenant's webhook processing should be capped, but the tenant should also have a global cap across every task they run. Queue gates let a run hold a concurrency slot in more than one queue at once.
 
@@ -236,12 +230,6 @@ await processWebhook.trigger(payload, {
 
 A run starts only when its home queue and every gate all have capacity, and it releases all of its slots together when it finishes or suspends.
 
-<Note>
-  Queue gates are enforced when the server has them enabled. If you self-host, set
-  `RUN_ENGINE_QUEUE_GATES_ENABLED=1`; servers without gates enabled accept the option but run
-  without it.
-</Note>
-
 ## Concurrency and subtasks
 
 When you trigger a task that has subtasks, the subtasks will not inherit the queue from the parent task. Unless otherwise specified, subtasks will run on their own queue