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

doc/man3/SSL_get_session.pod

@@ -48,6 +48,11 @@ SSL_SESSION object that cannot be used for resumption in TLSv1.3. It also
 enables applications to obtain information about all sessions sent by the
 server.
 
+A session will be automatically removed from the session cache and marked as
+non-resumable if the connection is not closed down cleanly, e.g. if a fatal
+error occurs on the connection or L<SSL_shutdown(3)> is not called prior to
+L<SSL_free(3)>.
+
 In TLSv1.3 it is recommended that each SSL_SESSION object is only used for
 resumption once.