default.yaml: add a comment on the probes that supports variable substitution

Signed-off-by: Norio Nomura <[email protected]>

Author: norio-nomura

examples/default.yaml

@@ -235,6 +235,7 @@ containerd:
 #   playbook: playbook.yaml
 
 # Probe scripts to check readiness.
+# The scripts can use the following template variables: {{.Home}}, {{.UID}}, {{.User}}, and {{.Param.Key}}
 # 🟢 Builtin default: null
 # probes:
 # # Only `readiness` probes are supported right now.