Document per-workload / ns annotation of shutdown enabling

[wmorgan]

#1816 did not capture the fact that you can enable this per workload or possibly per-ns.

From slack:

config.linkerd.io/proxy-admin-shutdown: enabled can be set on pod specs and, i believe, namespaces (would need to confirm that it works at the namespace level) to opt-in to the old behavior

[wmorgan]

See also https://github.com/linkerd/linkerd2/discussions/13105