[gnutls-devel] GnuTLS | Update docs for session ticket key rotation (!768)

[Development of GNU's TLS library]

Ander Juaristi commented on a discussion on doc/cha-gtls-app.texi:

>  A server supporting session tickets must generate ticket encryption
>  and authentication keys using @funcref{gnutls_session_ticket_key_generate}.
>  Those keys should be associated with the GnuTLS session using
> - at funcref{gnutls_session_ticket_enable_server}, and should be rotated regularly
> -(e.g., every few hours), to prevent them from becoming long-term keys which
> -if revealed could be used to decrypt all previous sessions.
> + at funcref{gnutls_session_ticket_enable_server}.
> +
> +GnuTLS will rotate these keys regularly. The key rotation interval can be specified with
> + at funcref{gnutls_db_set_cache_expiration}. Every such interval, new keys will be generated from the initial keys
> +that were first established using @funcref{gnutls_session_ticket_enable_server}. This is

I've rephrased it. I think it's clearer now.

```
A server supporting session tickets must generate ticket encryption
and authentication keys using @funcref{gnutls_session_ticket_key_generate}.
Those keys should be associated with the GnuTLS session using
@funcref{gnutls_session_ticket_enable_server}.

Those will be the initial keys, but GnuTLS will rotate them regularly. The key rotation interval
can be specified with @funcref{gnutls_db_set_cache_expiration}. Every such interval,
new keys will be generated from those initial keys. This is a necessary mechanism
to prevent the keys from becoming long-term keys
and as such preserve forward-secrecy in the issued session tickets. If no explicit key rotation interval
is provided, GnuTLS will rotate them every 18 hours by default.
```

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