diff --git a/src/doc/rustc-dev-guide/ci/linkcheck.sh b/src/doc/rustc-dev-guide/ci/linkcheck.sh
index 85573314c5fc..7149ab3a7f15 100755
--- a/src/doc/rustc-dev-guide/ci/linkcheck.sh
+++ b/src/doc/rustc-dev-guide/ci/linkcheck.sh
@@ -1,5 +1,6 @@
 #!/usr/bin/env bash
 
+# https://docs.travis-ci.com/user/environment-variables/#default-environment-variables
 if [ "$TRAVIS_EVENT_TYPE" = "cron" ] ; then # running in cron job
   FLAGS=""