feat(docs): add queue capacity property for fetch and lock long polling

Related to camunda/camunda-bpm-platform#4885

content/user-guide/process-engine/external-tasks.md

@@ -129,6 +129,22 @@ consider the following configuration snippet:
 <!-- ... -->
 ```
 
+#### Blocking Queue Capacity
+By default, the blocking queue of long-polling 'Fetch and Lock' requests has a capacity of 200, meaning that exceeding this number of requests will return an error. This value can be changed by setting the `fetch-and-lock-queue-capacity` property as a context parameter in the `engine-rest/WEB-INF/web.xml` file of the *engine-rest* artifact. If this property is absent or its value invalid, the default value will be used.
+
+Please consider the following configuration snippet:
+
+```xml
+<!-- ... -->
+
+<context-param>
+  <param-name>fetch-and-lock-queue-capacity</param-name>
+  <param-value>250</param-value>
+</context-param>
+
+<!-- ... -->
+```
+
 ## Java API
 
 The entry point to the Java API for external tasks is the `ExternalTaskService`. It can be accessed via `processEngine.getExternalTaskService()`.