feat(uart): documents UART_CLK_SRC_DEFAULT as option in header file

Author: SuGlider

cores/esp32/HardwareSerial.h

@@ -371,6 +371,7 @@ class HardwareSerial : public Stream {
   bool setMode(SerialMode mode);
   // Used to set the UART clock source mode. It must be set before calling begin(), otherwise it won't have any effect.
   // Not all clock source are available to every SoC. The compatible option are listed here:
+  // UART_CLK_SRC_DEFAULT      :: any SoC - it will set whatever IDF defines as the default UART Clock Source
   // UART_CLK_SRC_APB          :: ESP32, ESP32-S2, ESP32-C3 and ESP32-S3
   // UART_CLK_SRC_PLL          :: ESP32-C2, ESP32-C5, ESP32-C6, ESP32-C61, ESP32-H2 and ESP32-P4
   // UART_CLK_SRC_XTAL         :: ESP32-C2, ESP32-C3, ESP32-C5, ESP32-C6, ESP32-C61, ESP32-H2, ESP32-S3 and ESP32-P4