doc: clarify num_symbols in the tx done event data

Closes https://github.com/espressif/esp-idf/issues/10435

components/driver/include/driver/rmt_types.h

@@ -34,7 +34,8 @@ typedef struct rmt_encoder_t *rmt_encoder_handle_t;
  * @brief Type of RMT TX done event data
  */
 typedef struct {
-    size_t num_symbols; /*!< The number of transmitted RMT symbols (only one round is counted if it's a loop transmission) */
+    size_t num_symbols; /*!< The number of transmitted RMT symbols, including one EOF symbol, which is appended by the driver to mark the end of a transmission.
+                             For a loop transmission, this value only counts for one round. */
 } rmt_tx_done_event_data_t;
 
 /**

docs/en/api-reference/peripherals/rmt.rst

@@ -187,7 +187,7 @@ In the callback function, users can fetch the event specific data that is filled
 
 The TX done event data is defined in :cpp:type:`rmt_tx_done_event_data_t`:
 
-- :cpp:member:`rmt_tx_done_event_data_t::num_symbols` tells the number of transmitted RMT symbols. This also reflects the size of encoding artifacts.
+- :cpp:member:`rmt_tx_done_event_data_t::num_symbols` tells the number of transmitted RMT symbols. This also reflects the size of encoding artifacts. Please note, this value will count in the EOF symbol as well, which is appended by the driver, to mark the end of one transaction.
 
 The RX complete event data is defined in :cpp:type:`rmt_rx_done_event_data_t`: