QGrpcCallReply: rework documentation

Task-number: QTBUG-125406
Pick-to: 6.9 6.8
Change-Id: I373c39f99ee7ed00ebd9a84c29876d9131790a52
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

src/grpc/doc/src/qtgrpc-shared.qdocinc

@@ -15,3 +15,10 @@
 //! [qvariant-desc]
     Constructs a new QVariant from this object.
 //! [qvariant-desc]
+
+//! [rpc-lifetime-note]
+    \note Users are responsible for managing the unique RPC handlers returned
+    by the \c Client interface, ensuring their existence at least until the
+    \l{QGrpcOperation::} {finished} signal is emitted. After receiving this
+    signal, the handler can be safely reassigned or destroyed.
+//! [rpc-lifetime-note]

src/grpc/qgrpccallreply.cpp

@@ -15,18 +15,35 @@ using namespace Qt::StringLiterals;
 /*!
     \class QGrpcCallReply
     \inmodule QtGrpc
+    \brief The QGrpcCallReply class provides access in handling unary RPCs.
 
-    \brief The QGrpcCallReply class implements logic to handle \gRPC calls
-    from the \gRPC client side.
+    The QGrpcCallReply class provides the interface for handling unary remote
+    procedure calls (RPCs), which is one of the four \gRPC \l{Service
+    Methods}{service methods}.
 
-    The QGrpcCallReply object is owned by the client object that created it.
+    For a high-level overview, refer to the \l{Unary Calls} {Qt GRPC
+    Client Guide}.
+
+    \include qtgrpc-shared.qdocinc rpc-lifetime-note
 */
 
+/*!
+    \internal
+
+    Constructs a new QGrpcCallReply from an \a operationContext.
+
+    This is usually called by the generated client interface.
+
+    \sa QGrpcClientBase::call QAbstractGrpcChannel::call
+*/
 QGrpcCallReply::QGrpcCallReply(std::shared_ptr<QGrpcOperationContext> operationContext)
     : QGrpcOperation(std::move(operationContext))
 {
 }
 
+/*!
+    Destroys the QGrpcCallReply.
+*/
 QGrpcCallReply::~QGrpcCallReply() = default;
 
 bool QGrpcCallReply::event(QEvent *event)