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

[zackees]

Summary

Comment-only fix to the platforms/platformio.esp32p4.ini warning block added in PR #11.

The previous comment lumped ARDUINO_USB_MODE=1 together with ARDUINO_USB_CDC_ON_BOOT=1 as flags to avoid. After reading the Arduino-ESP32 core source (cores/esp32/{HWCDC,USBCDC,main}.cpp) and platform.txt, that conflation was wrong.

What ARDUINO_USB_MODE actually does

• It only selects which USB stack backs Serial when CDC_ON_BOOT=1. With CDC_ON_BOOT=0 (our default), Serial falls back to UART0 regardless of USB_MODE and the flag has no observable effect.
• =1 → HWCDC (passive ROM-backed USB-Serial/JTAG peripheral). Bounded ~2s write timeouts; does not wait for host enumeration.
• =0 → TinyUSB. This is the stack that blocks waiting for the host. It's the actual hang source when CDC_ON_BOOT=1 is paired with it.
• Espressif's platform.txt hard-codes =1 for C3/C5/C6/H2/C61. S3 defaults to =1; P4 defaults to =0.

Why no build-flag change

ARDUINO_USB_MODE doesn't matter for our envs because we don't set CDC_ON_BOOT=1. Leaving it unset matches each chip's upstream default and avoids cosmetic noise. Only the comment needed correcting — the comment was misleading future readers into thinking USB_MODE=1 was itself dangerous.

Test plan

• pio run -c platforms/platformio.esp32p4.ini still succeeds (no build-flag change)

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 7171a658-ccd0-40fd-9b28-84dcd3e4367d

📥 Commits

Reviewing files that changed from the base of the PR and between 021fde7 and efd54fd.

📒 Files selected for processing (1)

• platforms/platformio.esp32p4.ini

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/usb-mode-comment-accuracy

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