adding docs for task agent caching (#9436)

* adding docs for task agent caching

* missing CIRCLECI_RUNNER_ prefix

Author: soulchips

jekyll/_cci2/machine-runner-3-configuration-reference.adoc

@@ -257,6 +257,20 @@ runner:
   mode: single-task
 ```
 
+[#cache-task-agent]
+=== `runner.cache_task_agent`
+
+`$CIRCLECI_RUNNER_CACHE_TASK_AGENT`
+
+When set to true, machine runner will not clear the task-agent cache when the agent shuts down. On startup the machine agent will check for an already downloaded task agent and will use that task-agent unless there is a newer version of task-agent available for download. This feature is off by default. This may be particularly useful for Windows which relies on `single-task` mode.
+
+Example:
+
+```yaml
+runner:
+  cache_task_agent: true
+```
+
 [#runner-max-run-time]
 === `runner.max_run_time`