QSslConfiguration: add documentation for HTTP/2 alpn variable

It was missing for a long time

Pick-to: 6.7 6.5
Change-Id: I209838a3c7b45c8c2e8da21efea306fafddea891
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Mate Barany <mate.barany@qt.io>

src/network/ssl/qsslconfiguration.cpp

@@ -104,6 +104,12 @@ const char QSslConfiguration::NextProtocolHttp1_1[] = "http/1.1";
     Protocol Negotiation.
 */
 
+/*!
+    \variable QSslConfiguration::ALPNProtocolHTTP2
+    \brief The value used for negotiating HTTP 2 during the Application-Layer
+    Protocol Negotiation.
+*/
+
 /*!
     Constructs an empty SSL configuration. This configuration contains
     no valid settings and the state will be empty. isNull() will