boards/jiminy-mega256rfr2: cleanup documentation

- Cleanup brief description to be consistent with other boards
- Move general documentation in doc.txt

boards/jiminy-mega256rfr2/doc.txt

+/**
+ * @defgroup    boards_jiminy-mega256rfr2 Jiminy- Mega256rfr2
+ * @ingroup     boards
+ * @brief       Support for the Jiminy Mega 256rfr2 board
+ */
\ No newline at end of file

boards/jiminy-mega256rfr2/include/board.h

@@ -7,9 +7,7 @@
  */
 
 /**
- * @defgroup    boards_jiminy-mega256rfr2 Jiminy- Mega256rfr2
- * @ingroup     boards
- * @brief       Board specific files for the Jiminy Mega 256rfr2 board.
+ * @ingroup     boards_jiminy-mega256rfr2
  * @{
  *
  * @file