docs(esp32p4): correct USB CDC warning — only CDC_ON_BOOT is toxic (#12)

The previous comment lumped ARDUINO_USB_MODE=1 in with
ARDUINO_USB_CDC_ON_BOOT=1 as a flag to avoid. That was wrong.

Per the Arduino-ESP32 core source (cores/esp32/{HWCDC,USBCDC,main}.cpp,
platform.txt):

- ARDUINO_USB_MODE only selects HWCDC vs TinyUSB when CDC_ON_BOOT=1.
  With CDC_ON_BOOT=0 (our default) Serial falls back to UART0 regardless
  of USB_MODE — the flag has no observable effect.
- =1 selects HWCDC (passive ROM-backed USB-Serial/JTAG). Bounded ~2s
  write timeouts; does not wait for host enumeration.
- =0 selects TinyUSB, which IS the stack that blocks waiting for the
  host. P4's upstream default is 0; C3/C5/C6/H2/C61 force =1 via
  platform.txt.

Update the comment to point users at the actual toxic flag
(CDC_ON_BOOT=1) and explain that USB_MODE is left unset because it
doesn't matter while CDC-on-boot is disabled.

No build-flag changes; comment-only.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: zackees

platforms/platformio.esp32p4.ini

@@ -19,9 +19,15 @@ board_build.partitions = huge_app.csv
 ; stalls during execution, silently corrupting LED protocol timing.
 ; See FastLED src/platforms/esp/32/drivers/parlio/ for the canonical chipset list.
 ;
-; Do NOT add -DARDUINO_USB_CDC_ON_BOOT=1 or -DARDUINO_USB_MODE=1 here. They
-; wire Serial to the USB-CDC interface, which only enumerates when a USB host
-; is connected — boards powered from a wall adapter then hang in setup().
+; Do NOT add -DARDUINO_USB_CDC_ON_BOOT=1 here. It wires Serial to the USB-CDC
+; interface, which only enumerates when a USB host is connected — sketches
+; that touch Serial in setup() (especially `while (!Serial) {}`) then hang
+; when the board is powered from a wall adapter instead of a PC.
+;
+; ARDUINO_USB_MODE selects HWCDC vs TinyUSB *when CDC-on-boot is enabled*;
+; it has no effect on Serial routing while CDC_ON_BOOT=0, so we leave it
+; unset here. P4's upstream default is 0 (TinyUSB); C3/C5/C6/H2 force it to
+; 1 (HWCDC) via platform.txt.
 build_flags =
   -DCONFIG_PARLIO_TX_ISR_HANDLER_IN_IRAM=1
   -DCONFIG_PARLIO_TX_ISR_CACHE_SAFE=1