Update Loading benchmarks document

Running loading benchmarks on Pinpoint with a valid configuration is a
bit tricky, so this adds some guides for providing correct arguments for
Pinpoint.

Bug: 1114724
Change-Id: I64e3919b1e3d3e94b8c815e8a747f392f3da4854
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352291Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797581}

docs/speed/benchmark/harnesses/loading.md

@@ -18,11 +18,27 @@ corpus of top 10 thousands URLs from Alexa. Unlike the other two loading
 benchmarks which are run continuously on the perf waterfall, this benchmark is
 triggered on-demand only.
 
-# Running the tests remotely
+## Running the tests remotely
 
 If you're just trying to gauge whether your change has caused a loading
-regression, you can either run `loading.desktop` and `loading.mobile` through
-[perf try job](https://chromium.googlesource.com/chromium/src/+/master/docs/speed/perf_trybots.md) or you can run `loading.cluster_telemetry` through
+regression, you can run the loading benchmarks through perf try job or through
+Cluster Telemetry service. You can specify a Gerrit patch for them and compare
+the results with and without the patch applied.
+
+### Using Perf Try Bots
+[Perf Try Bots](https://chromium.googlesource.com/chromium/src/+/master/docs/speed/perf_trybots.md)
+lets you run only single story of `loading.desktop` or `loading.mobile`, because
+an entire loading benchmark is too large to run as a perf try job. You should
+only set the `Story` field, and leave the `Story Tags` field blank. Invalid
+combinations of `Story` and `Story Tags` will result in "zero Histograms" result
+page.
+
+See
+[Understanding the loading test cases](#understanding-the-loading-test-cases)
+for story name conventions.
+
+### Using Cluster Telemetry Service
+You can run `loading.cluster_telemetry` (top 10k pages) through
 [Cluster Telemetry service](https://ct.skia.org/) (Cluster Telemetry is for
 Googler only).