[autotest] Add documentation to global config for ES_HOST

Add doc stating chromeos-lab-graphite is test server.

BUG=None
TEST=Tried querying the server, got something back.
Change-Id: I81ee9fe2461518b6fb0d9ed991ad6735f0cfb144
Reviewed-on: https://chromium-review.googlesource.com/207801
Tested-by: Michael Liang <michaelliang@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Michael Liang <michaelliang@chromium.org>
diff --git a/global_config.ini b/global_config.ini
index 7f032c3..fe32778 100644
--- a/global_config.ini
+++ b/global_config.ini
@@ -206,6 +206,7 @@
 extra_servers: chromeos-mcp
 
 # ES_HOST's hostname is chromeos-lab-graphite.cbf.corp.google.com
+# This es server is used for testing purposes only.
 ES_HOST: 172.25.61.45
 ES_PORT: 9200
 ES_UDP_PORT: 9700