esb: Update Doxygen documentation for tx_output_power

Updated Doxygen documentation for tx_output_power parameter in
esb_config structure.

Ref: NCSDK-30436

Signed-off-by: Marcin Jelinski <marcin.jelinski@nordicsemi.no>

Author: maje-emb

include/esb.h

@@ -322,7 +322,9 @@ struct esb_config {
 	/* General RF parameters */
 	enum esb_bitrate bitrate;		/**< Bitrate mode. */
 	enum esb_crc crc;			/**< CRC mode. */
-	int8_t tx_output_power;	                /**< Radio TX power. */
+	int8_t tx_output_power;			/**< Radio TX power.
+						  *  Output power in dBm.
+						  */
 
 	uint16_t retransmit_delay; /**< The delay between each retransmission of
 				  *  unacknowledged packets.