Doc: edit QWebEngineCookieStoreClient::FilterRequest docs

- Add \brief commands.
- Each \variable command needs to be in a separate help topic.
- Edit the language

Change-Id: Iac9b57ceaca5d3decce83305b8d3d39d54eb09dd
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>