Doc: clean up QWebEngineScript docs

- Add missing documentation
- Remove unnecessary \brief, \param, and \return commands
- Edit for language and style

Change-Id: Iff9ddc56ab32af783831c3a962b0967fcbddde24
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>