Add class and enum documentation to QWebEngineCertificateError

Without the class description the methods documentation was never
generated.

Change-Id: I0d7232e8f5cc6765908cb28b77d0fe0f514d359d
Task-number: QTBUG-42996
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>