Add note for containerd oom_score (#9384)

When we saw 0 as the default value of containerd_oom_score, we had
a question why the value was not -999.
This adds the note to explain it.

roles/container-engine/containerd/defaults/main.yml

@@ -2,6 +2,9 @@
 containerd_storage_dir: "/var/lib/containerd"
 containerd_state_dir: "/run/containerd"
 containerd_systemd_dir: "/etc/systemd/system/containerd.service.d"
+# The default value is not -999 here because containerd's oom_score_adj has been
+# set to the -999 even if containerd_oom_score is 0.
+# Ref: https://github.com/kubernetes-sigs/kubespray/pull/9275#issuecomment-1246499242
 containerd_oom_score: 0
 
 # containerd_default_runtime: "runc"