Document wait time between attempts in `ssh_handshake_attempts`

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#### Description

$subj or make it configurable.
Had to look into [packer-plugin-sdk sources](https://github.com/hashicorp/packer-plugin-sdk/blob/main/communicator/step_connect_ssh.go#L234) to check what is wait time. (2 seconds at the moment I'm writing this)

#### Use Case(s)

To better understand what to set in `ssh_handshake_attempts`, e.g. `expected_wait_time_in_seconds/2`.

Sometimes if cloud-init adds a public key it may take some time, e.g. when it's datasources have to wait for some external metadata. So first minute after sshd daemon is up, no one will be able to authorize since public key is not added to `.ssh/authorized_keys` yet. In such condition packer may fail since 10 attempts will happen in just 20 seconds.