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>
(Merged from https://github.com/openssl/openssl/pull/6054)

(cherry picked from commit 0abb903b)