Doc: QRhi corrections

* There's no convenience header for the `QRhi` class, so add the
  `\inheaderfile` command to specify the correct include.
* The `QRhi` class belongs to the `QtGuiPrivate` module; use the
  correct name.
* QDoc needs to know about the QtGuiPrivate module. Add basic
  documentation for the private module.

Task-number: QTBUG-121855
Task-number: QTBUG-121991
Change-Id: I1ffd729a5f9c928f4cf91ce7127f40c7733fc42f
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>

src/gui/doc/src/qtgui.qdoc

@@ -14,6 +14,23 @@
     The \l{Qt GUI} page contains information about how to use the module.
 */
 
+/*!
+    \module QtGuiPrivate
+    \title Qt GUI Private C++ Classes
+    \qtcmakepackage Gui
+    \qtvariable gui-private
+
+    \brief Provides access to private GUI functionality.
+
+    Use the following CMake commands in your \c {CMakeLists.txt} to access
+    private Qt GUI APIs:
+
+    \badcode
+    find_package(Qt6 REQUIRED COMPONENTS Gui)
+    target_link_libraries(mytarget PRIVATE Qt6::GuiPrivate)
+    \endcode
+*/
+
 /*!
     \page qtgui-index.html
     \title Qt GUI

src/gui/rhi/qrhi.cpp

@@ -29,7 +29,8 @@ Q_LOGGING_CATEGORY(QRHI_LOG_INFO, "qt.rhi.general")
 /*!
     \class QRhi
     \ingroup painting-3D
-    \inmodule QtGui
+    \inmodule QtGuiPrivate
+    \inheaderfile rhi/qrhi.h
     \since 6.6
 
     \brief Accelerated 2D/3D graphics API abstraction.