qdoc: Document C++ functions that are invokable

This update lets clang report to qdoc whenever Q_INVOKABLE
appears in a C++ function declaration so that qdoc can add
a note to the function's documentation telling the reader
that the function is invokable via the meta-object system
and from QML.

Task-number: QTBUG-59083
Change-Id: I9b62ba1c9159a1ec43c6a59e576efdb71bff62e5
Reviewed-by: Martin Smith <martin.smith@qt.io>