bootloader: add note about secure download mode for ESP32 target

Closes IDFGH-5857
Closes https://github.com/espressif/esp-idf/issues/7557

components/bootloader/Kconfig.projbuild

@@ -892,6 +892,8 @@ menu "Security features"
                 It is also possible to enable secure download mode at runtime by calling
                 esp_efuse_enable_rom_secure_download_mode()
 
+                Note: Secure Download mode is not available for ESP32 (includes revisions till ECO3).
+
         config SECURE_INSECURE_ALLOW_DL_MODE
             bool "UART ROM download mode (Enabled (not recommended))"
             help