docs: Add internal documentation for QGrpcCommonOptions

... to avoid qdoc warnings about it being undocumented.

Pick-to: 6.10
Change-Id: Ic092c26d7e43dd9a51646cc6edf6f418bc8857ec
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

src/grpc/qgrpccommonoptions.cpp

@@ -40,6 +40,14 @@ QHash<QByteArray, QByteArray> QGrpcCommonOptions::metadata() &&
     return std::move(m_metadata);
 }
 
+/*!
+    \internal
+    \class QGrpcCommonOptions
+    \inmodule QtGrpc
+    \brief Shared implementation of QGrpcCallOptions and QGrpcChannelOptions
+    functionality.
+*/
+
 /*!
 //! [set-metadata]
     Sets the \a metadata and returns a reference to the updated object.