doc: cpu: fix doxygen

cpu/atmega_common/avr-libc-extra/errno.h

@@ -55,7 +55,7 @@ extern "C" {
 #endif
 
 /**
- * @brief   Export errno value
+ * @brief number of last error
  */
 extern int errno;
 

cpu/cc26x0/doc.txt

@@ -5,7 +5,7 @@
  */
 
 /**
- * @defgroup        cpu_specific_peripheral_memory_map CC26x0 peripheral memory map
+ * @defgroup        cpu_specific_peripheral_memory_map Texas Instruments CC26x0 peripheral memory map
  * @ingroup         cpu
  */
 

cpu/x86/include/x86_rtc.h

@@ -50,7 +50,8 @@ typedef union x86_rtc_data {
         uint8_t second, minute, hour;
         uint8_t day, month, year, century;
     };
-    uint64_t timestamp;
+    uint64_t timestamp; /**< The full timestamp as a 64bit integer
+                              representation */
 } x86_rtc_data_t;
 
 #define RTC_REG_SECOND         (0x00)