CURLINFO_TLS_SSL_PTR.3: Clarify SSL pointer availability

Bug: https://curl.haxx.se/mail/lib-2016-04/0126.html
Reported-by: Bru Rom

docs/libcurl/opts/CURLINFO_TLS_SSL_PTR.3

@@ -92,7 +92,8 @@ SSL *
 .RE
 
 If the \fIinternals\fP pointer is NULL then either the SSL backend is not
-supported or an SSL session has not yet been established.
+supported, an SSL session has not yet been established or the connection is no
+longer associated with the easy handle (eg curl_easy_perform has returned).
 .SH LIMITATIONS
 \fBThis option has some limitations that could make it unsafe when it comes to
 the manual verification of certificates.\fP