doc: fix UART doxygen docu

- move detailed description to be visible on respective module page
    - see and compare other periph interfaces, i.e., GPIO, PWM ...

drivers/include/periph/uart.h

@@ -10,7 +10,6 @@
  * @defgroup    drivers_periph_uart UART
  * @ingroup     drivers_periph
  * @brief       Low-level UART peripheral driver
- * @{
  *
  * This is a basic UART (Universal Asynchronous Receiver Transmitter) interface
  * to allow platform independent access to the MCU's serial communication abilities.
@@ -35,6 +34,8 @@
  * in RIOT which is used for standard input/output functions like `printf()` or
  * `puts()`.
  *
+ * @{
+ *
  * @file
  * @brief       Low-level UART peripheral driver interface definition
  *