Doc: be more explicit about need to set expected SSL cert in errors

Note added in QNetworkReply and QSslSocket documentation.

Task-number: QTBUG-16770
Change-Id: I2dd8cfb913ec29a96b5465a905cd213713b8d537
Reviewed-by: Richard J. Moore <rich@kde.org>
(cherry picked from qtbase/34590e84d4aaceb5874d8acb6a2b7a95c153cd6a)