Update description about proxy wait

[mikutas]

While reading https://linkerd.io/2.16/tasks/graceful-shutdown/
and https://linkerd.io/2.16/reference/proxy-configuration/ ,
I was a little confused about config.alpha.linkerd.io/proxy-wait-before-exit-seconds.

So let me suggest new description about it.

I think it's important when SIGTERM is received.

When the Linkerd proxy sidecar receives this signal, it will immediately begin a graceful shutdown where it refuses all new requests and allows existing requests to complete before shutting down.