Document that the Windows version in the user-agent might be wrong

Chromium uses GetVersionEx() to determine the Windows version that is
reported in the default user agent. GetVersionEx() however always
reports "Windows NT 6.2" for newer Windows versions, unless the
executable's manifest file marks the newer version as supported.

Since this is a common question we document this in the httpUserAgent
accessors.

Task-number: QTBUG-56472
Change-Id: I4698cd659f5552b92ef925c198f39326fcb936f3
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/webengine/api/qquickwebengineprofile.cpp

@@ -478,6 +478,10 @@ void QQuickWebEngineProfile::setCachePath(const QString &path)
     \qmlproperty string WebEngineProfile::httpUserAgent
 
     The user-agent string sent with HTTP to identify the browser.
+
+    \note On Windows 8.1 and newer, the default user agent will always report
+    "Windows NT 6.2" (Windows 8), unless the application does contain a manifest
+    that declares newer Windows versions as supported.
 */
 
 /*!

src/webenginewidgets/api/qwebengineprofile.cpp

@@ -365,6 +365,10 @@ void QWebEngineProfile::setCachePath(const QString &path)
 /*!
     Returns the user-agent string sent with HTTP to identify the browser.
 
+    \note On Windows 8.1 and newer, the default user agent will always report
+    "Windows NT 6.2" (Windows 8), unless the application does contain a manifest
+    that declares newer Windows versions as supported.
+
     \sa setHttpUserAgent()
 */
 QString QWebEngineProfile::httpUserAgent() const