cpukit: Document the C++ API files

The headers thread.hpp and error.hpp wrote @ingroup RTEMSC++.  Doxygen
stops a group name at the first character which is not a letter, a digit
or an underscore.  It therefore looked for a group named RTEMSC.  No
file defines either name.  The two sources in librtemscxx had no Doxygen
block at all.  The briefs of the two headers named a topic, not the
content of the file.

Define RTEMSAPICXX under RTEMSAPI for the two headers and RTEMSImplCXX
under RTEMSImpl for the two sources.  Give the sources a file block.
Write the four briefs in the style of the tree.

Updates #5717.

Assisted-by: Claude:claude-opus-5 claude-code
Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>

cpukit/include/rtems/error.hpp

@@ -3,9 +3,9 @@
 /**
  * @file
  *
- * @ingroup RTEMSC++
+ * @ingroup RTEMSAPICXX
  *
- * @brief RTEMS Error exception.
+ * @brief This header file provides the C++ error exception support.
  *
  * Provide an error exception for RTEMS errors.
  */

cpukit/include/rtems/thread.hpp

@@ -3,9 +3,10 @@
 /**
  * @file
  *
- * @ingroup RTEMSC++
+ * @ingroup RTEMSAPICXX
  *
- * @brief C++ standard thread support with thread attribute control.
+ * @brief This header file provides the C++ thread support with control of
+ *   the thread attributes.
  *
  * Provide a way to create a thread in C++ with attributes that let
  * you control the real-time embedded parameters need to run
@@ -42,6 +43,16 @@
 #if !defined(RTEMS_THREAD_HPP)
 #define RTEMS_THREAD_HPP
 
+/**
+ * @defgroup RTEMSAPICXX C++ API
+ *
+ * @ingroup RTEMSAPI
+ *
+ * @brief This group contains the C++ API of RTEMS.
+ *
+ * @{
+ */
+
 #include <functional>
 #include <iostream>
 #include <string>
@@ -468,4 +479,6 @@ inline std::basic_ostream<C, T>& operator<<(std::basic_ostream<C, T>& out,
 }; // namespace thread
 }; // namespace rtems
 
+/** @} */
+
 #endif

cpukit/librtemscxx/error.cpp

@@ -1,5 +1,14 @@
 /* SPDX-License-Identifier: BSD-2-Clause */
 
+/**
+ * @file
+ *
+ * @ingroup RTEMSImplCXX
+ *
+ * @brief This source file contains the implementation of the C++ error
+ *   support.
+ */
+
 /*
  * Copyright (C) 2020 Chris Johns <chris@contemporary.software>
  *

cpukit/librtemscxx/thread.cpp

@@ -1,5 +1,14 @@
 /* SPDX-License-Identifier: BSD-2-Clause */
 
+/**
+ * @file
+ *
+ * @ingroup RTEMSImplCXX
+ *
+ * @brief This source file contains the implementation of the C++ thread
+ *   support.
+ */
+
 /*
  * Copyright (C) 2020 Chris Johns <chris@contemporary.software>
  *
@@ -29,6 +38,14 @@
 #define _GNU_SOURCE
 #endif
 
+/**
+ * @defgroup RTEMSImplCXX C++ Support
+ *
+ * @ingroup RTEMSImpl
+ *
+ * @brief This group contains the implementation of the C++ API of RTEMS.
+ */
+
 #include <system_error>
 
 #include <rtems/error.hpp>