Document when a session gets removed from cache

Document the fact that if a session is not closed down cleanly then the
session gets removed from the cache and marked as non-resumable.

Fixes #4720

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/6053)