[ci] explain why check_fmt.sh is not silenced

githooks/pre-push

@@ -1,6 +1,8 @@
 #!/usr/bin/env bash
 set -eo pipefail
 echo "Running pre-push git hook: $0"
+# forego redirecting stdout to /dev/null on check_fmt.sh because the output
+# from cargofmt is useful (and the good stuff is not delivered by stderr)
 ./ci/check_fmt.sh
 ./ci/check_job_dependencies.sh > /dev/null
 ./ci/check_msrv.sh > /dev/null