doxygen: cleanup code for group_object_management

The cleanup involved three parts:

- Merging the standalone "/**@{*/" into
  "@addtogroup group_object_management"

- Changing "/**@}*/" to "/** @} group_object_management */",
  adding the group name makes it easier to find the matching "@{" part.

- Deleting the "@addtogroup" directive in "include/rtthread.h"
  because this header file doesn't have doxygen comments.
  All doxygen comments for function bodies are defined in "src/object.c".

Plus minor cleanup for group_hook in src/object.c.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

include/rtdef.h

@@ -267,10 +267,9 @@ typedef int (*init_fn_t)(void);
 
 /**
  * @addtogroup group_object_management
+ * @{
  */
 
-/**@{*/
-
 /*
  * kernel object macros
  */
@@ -512,7 +511,7 @@ struct rt_object_information
 #define RT_OBJECT_HOOKLIST_CALL(name, argv)
 #endif /* RT_USING_HOOKLIST */
 
-/**@}*/
+/** @} group_object_management */
 
 /**
  * @addtogroup group_clock_management