doc: efm32: prs: enable peripheral_apis style

Include _basic_ description of unusual acronyms

lib/efm32/common/prs_common.c

@@ -1,3 +1,10 @@
+/** @addtogroup prs_file PRS peripheral API
+ * @ingroup peripheral_apis
+ * @brief EFM32 Peripheral Reflex System (PRS).
+ * The Peripheral Reflex System (PRS) system is a network which allows the
+ * different peripheral modules to communicate directly with each other
+ * without involving the CPU.
+ */
 /*
  * This file is part of the libopencm3 project.
  *
@@ -19,6 +26,8 @@
 
 #include <libopencm3/efm32/prs.h>
 
+/**@{*/
+
 /**
  * Enable PRS output to GPIO.
  * @param[in] ch Channel (use PRS_CHx)
@@ -138,3 +147,5 @@ void prs_set_signal(enum prs_ch ch, uint32_t signal)
 	PRS_CHx_CTRL(ch) = (PRS_CHx_CTRL(ch) & ~PRS_CH_CTRL_SIGSEL_MASK)
 			   | signal;
 }
+
+/**@}*/