docs: update stateful resumption doc

[jouho]

Resolved issues:

When attempting to perform session resumption using session caching, the TLS connection must be closed gracefully. If not, the errored session cache is deleted from the server, leading to a failed resumption and triggering a full handshake instead. We should document this behavior and update the usage guide.

Description of changes:

Updated usage guide and documented stateful resumption behavior.

Call-outs:

Stateless session resumption behaves differently as it is able to resume a session without graceful TLS shutdown. This shouldn't have security implications as stated in RFC5077:

In the analysis within this paper, failure to invalidate
sessions does not pose a security risk. This is because the TLS
handshake uses a non-reversible function to derive keys for a session
so information about one session does not provide an advantage to
attack the master secret or a different session.

Testing:

No functional changes

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[goatgoose]

also meta-nit: this seems more like a docs PR than a chore