Add docs hint that ssh_public_key should be base64 encoded

[itsamemarkus]

Documentation link

https://galaxy.ansible.com/ui/repo/published/paloaltonetworks/panos/content/module/panos_administrator/

https://galaxy.ansible.com/ui/repo/published/paloaltonetworks/panos/content/module/panos_admin/

Describe the problem

It is not intuitively obvious that the ssh_public_key has to be base64 encoded

Suggested fix

add a hint that the public key should be base64 encoded