doxygen: refine core_sync grouping

- Move `@defgroup core_sync` group definition to doc.txt
- Adapt usage of `@ingroup core_sync` accordingly

core/doc.txt

@@ -25,3 +25,9 @@
  * @ingroup     core
  * @brief       Configuration data and startup code for the kernel
  */
+
+ /**
+  * @defgroup    core_sync Thread Synchronization
+  * @ingroup     core
+  * @brief       Thread synchronization mechanisms of the kernel
+  */

core/include/cond.h

@@ -5,12 +5,9 @@
  */
 
 /**
+ * @defgroup    core_sync_cond Condition Variable
+ * @ingroup     core_sync
  * @brief       Condition variable for thread synchronization
- * @ingroup     core, core_sync
- * @{
- *
- * @file
- * @brief       RIOT synchronization API
  *
  * This file contains a condition variable with Mesa-style semantics.
  *
@@ -129,6 +126,11 @@
  * Note that this could actually be written with a single condition variable.
  * However, the example includes two for didactic reasons.
  *
+ * @{
+ *
+ * @file
+ * @brief       Condition variable for thread synchronization
+ *
  * @author      Sam Kumar <samkumar@berkeley.edu>
  */
 

core/include/mutex.h

@@ -8,13 +8,13 @@
  */
 
 /**
- * @defgroup    core_sync Synchronization
+ * @defgroup    core_sync_mutex Mutex
+ * @ingroup     core_sync
  * @brief       Mutex for thread synchronization
- * @ingroup     core
  * @{
  *
  * @file
- * @brief       RIOT synchronization API
+ * @brief       Mutex for thread synchronization
  *
  * @author      Kaspar Schleiser <kaspar@schleiser.de>
  */

core/include/rmutex.h

@@ -7,12 +7,14 @@
  */
 
 /**
+ * @defgroup    core_sync_rmutex Recursive Mutex
  * @ingroup     core_sync
  * @brief       Recursive Mutex for thread synchronization
+ *
  * @{
  *
  * @file
- * @brief       RIOT synchronization API
+ * @brief       Recursive Mutex for thread synchronization
  *
  * @author      Martin Elshuber <martin.elshuber@theobroma-systems.com>
  *