Document endpoint selection for ZMK Studio to work

[caksoylar]

I believe the selected endpoint (USB vs. BLE) needs to match how the ZMK Studio client is connected to, in order for Studio to work as expected. This caveat should be documented, probably in https://zmk.dev/docs/features/studio.

One question: Does a fallback work for this? e.g. if BLE priority is selected, ZMK Studio is connected via USB, and keyboard isn't currently connected over BLE, does it still require selecting USB endpoint priority via &out OUT_USB?