Document that we discourage event processing from slots connected to QNetworkReply

The QNetworkReply signals like readyRead() should have documented that the user should not

• spin a QEventLoop
• call any processEvents()
• Display any dialog
  or otherwise do anything that processes events.

The only exceptions are obviously authenticationRequired(), proxyAuthenticationRequired() and sslErrors() since those are often used to display a dialog to the user.