doc: improve TLS/SSL introduction
TLS/SSL is not a public key infrastructure. It is a set of protocols that use a public key infrastructure. Also add the purpose of TLS/SSL (secure communication) to the very beginning.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
TLS/SSL is not a public key infrastructure. It is a set of protocols that use a public key infrastructure. Also add the purpose of TLS/SSL (secure communication) to the very beginning.