doc: Clarify that calling SSL_set_session(ssl, NULL) is a correct use…

… case.

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from openssl#18101)

doc/man3/SSL_set_session.pod

@@ -21,7 +21,8 @@ with the L<SSL_session_reused(3)> call.
 
 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. If that old
+a connection), SSL_SESSION_free() will be called for that session.
+This is also the case when B<session> is a NULL pointer. 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