sip.conf: tlsclientmethod is using sslv23 as default.

When 'tlsclientmethod' is not specified in sip.conf, chan_sip uses the OpenSSL
SSLv23_method. This was documented incorrectly in the file sip.conf.sample.

SSLv23_method got its name in the 90s. Today, with OpenSSL 1.0.2, this method
enables (just) the secure TLSv1.0 and TLSv1.2. Or stated differently, that
function should have been called 'secure_method' or 'automatic_method' back in
the 90s.

Consequently please, specify 'tlsclientmethod=tlsv1' in your sip.conf only if
you face a server which has problems like not falling back to TLSv1.0
automatically.

ASTERISK-24425

Change-Id: I502ce6146b4504cadfd3973af8d6ec3994f54fa3

configs/samples/sip.conf.sample

@@ -611,7 +611,16 @@ srvlookup=yes                   ; Enable DNS SRV lookups on outbound calls
 ;
 ;tlsclientmethod=tlsv1     ; values include tlsv1, sslv3, sslv2.
                            ; Specify protocol for outbound client connections.
-                           ; If left unspecified, the default is sslv2.
+                           ; If left unspecified, the default is the general-
+                           ; purpose version-flexible SSL/TLS method (sslv23).
+                           ; With that, the actual protocol version used will
+                           ; be negotiated to the highest version mutually
+                           ; supported by Asterisk and the remote server, i.e.
+                           ; TLSv1.2. The supported protocols are listed at
+                           ; http://www.openssl.org/docs/ssl/SSL_CTX_new.html
+                           ; SSLv2 and SSLv3 are disabled within Asterisk.
+                           ; Your distribution might have changed that list
+                           ; further.
 ;
 ;--------------------------- SIP timers ----------------------------------------------------
 ; These timers are used primarily in INVITE transactions.