run-unit-test-docker: add comment for TEST_ONLY

The NO_FORMAT_CODE option is in the comment block. It also makes sense
to add the TEST_ONLY option to that comment block.

Tested: tested locally though it only changes a comment.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I49f48e813c633bd89b9c82df128e84e3c59c8993
diff --git a/run-unit-test-docker.sh b/run-unit-test-docker.sh
index fd3c3f7..e00f981 100755
--- a/run-unit-test-docker.sh
+++ b/run-unit-test-docker.sh
@@ -21,6 +21,7 @@
 #                    used if input branch not provided or not found
 #   dbus_sys_config_file: Optional, with the default being
 #                         `/usr/share/dbus-1/system.conf`
+#   TEST_ONLY:       Optional, do not run analysis tools
 #   NO_FORMAT_CODE:  Optional, do not run format-code.sh
 #   EXTRA_UNIT_TEST_ARGS:  Optional, pass arguments to unit-test.py
 #   INTERACTIVE: Optional, run a bash shell instead of unit-test.py