Update mojo_benchmark documentation.

mojo/devtools/common/docs/mojo_benchmark.md

 # mojo_benchmark
 
 `mojo_benchmark` allows you to run performance tests for any Mojo application
 that participates in the [tracing
 ecosystem](https://github.com/domokit/mojo/blob/master/mojo/services/tracing/public/interfaces/tracing.mojom)
 with no app changes required.
 
 The script reads a list of benchmarks to run from a file, runs each benchmark in
 controlled caching conditions with tracing enabled and performs specified
 measurements on the collected trace data.
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 ```
 
 The following types of measurements are available:
 
  - `time_until` - measures time until the first occurence of the specified event
  - `avg_duration` - measures the average duration of all instances of the 
     specified event
 
 ## Caching
 
-The script runs each benchmark twice. The first run (**cold start**) clears the
-following caches before start:
+The script runs each benchmark twice. The first run (**cold start**) clears
+caches of the following apps on startup:
 
- - url_response_disk_cache
+ - network_service.mojo
+ - url_response_disk_cache.mojo
 
 The second run (**warm start**) runs immediately afterwards, without clearing
 any caches.
 
 ## Time origin
 
 The underlying benchmark runner records the time origin just before issuing the
 connection call to the application being benchmarked. Results of `time_until`
 measurements are relative to this time.
 
 ## Example
 
 For an app that records a trace event named "initialized" in category "my_app"
 once its initialization is complete, we can benchmark the initialization time of
 the app (from the moment someone tries to connect to it to the app completing
 its initialization) using the following benchmark file:
 
 ```python
 benchmarks = [
   {
     'name': 'My app initialization',
     'app': 'https://my_domain/my_app.mojo',
     'duration': 10,
     'measurements': [
       'time_until/my_app/initialized',
     ],
   },
 ]
 ```