Update the SSL_set_session() documentation

Update the SSL_set_session() documentation to reflect the fact that old
bad sessions are removed from the cache if necessary.

Reviewed-by: Rich Salz <rsalz@openssl.org>

doc/ssl/SSL_set_session.pod

diff --git a/doc/ssl/SSL_set_session.pod b/doc/ssl/SSL_set_session.pod
index 0a8a208..1de533f 100644
--- a/doc/ssl/SSL_set_session.pod
+++ b/doc/ssl/SSL_set_session.pod
@@ -21,7 +21,11 @@
 
 If there is already a session set inside B<ssl> (because it was set with
 SSL_set_session() before or because the same B<ssl> was already used for
-a connection), SSL_SESSION_free() will be called for that session.
+a connection), SSL_SESSION_free() will be called for that session. If that old
+session is still B<open>, it is considered bad and will be removed from the
+session cache (if used). A session is considered open, if L<SSL_shutdown(3)> was
+not called for the connection (or at least L<SSL_set_shutdown(3)> was used to
+set the SSL_SENT_SHUTDOWN state).
 
 =head1 NOTES