Clarify that SSL_shutdown() must not be called after a fatal error

=item SSL_ERROR_SYSCALL

Some non-recoverable I/O error occurred.

The OpenSSL error queue may contain more information on the error.

For socket I/O on Unix systems, consult B<errno> for details.

Some non-recoverable, fatal I/O error occurred. The OpenSSL error queue may

contain more information on the error. For socket I/O on Unix systems, consult

B<errno> for details. If this error occurs then no further I/O operations should

be performed on the connection and SSL_shutdown() must not be called.

=item SSL_ERROR_SSL

A failure in the SSL library occurred, usually a protocol error.  The

OpenSSL error queue contains more information on the error.

A non-recoverable, fatal error in the SSL library occurred, usually a protocol

error.  The OpenSSL error queue contains more information on the error. If this

error occurs then no further I/O operations should be performed on the

connection and SSL_shutdown() must not be called.

=back

a currently open session is considered closed and good and will be kept in the

session cache for further reuse.

Note that SSL_shutdown() must not be called if a previous fatal error has

occurred on a connection i.e. if SSL_get_error() has returned SSL_ERROR_SYSCALL

or SSL_ERROR_SSL.

The shutdown procedure consists of 2 steps: the sending of the "close notify"

shutdown alert and the reception of the peer's "close notify" shutdown

alert. According to the TLS standard, it is acceptable for an application