include: drivers: uart: remove incorrect comments about CDC ACM UART

The CDC ACM implementation provides a virtual UART interface
that is used by various samples and subsystems in a way that is
compatible with the real UART interface.

Commit cc1b2c70
("uart: doc: Add special case for virtual UART")
added a questionable comment to uart_fifo_fill() API
description in an attempt to fix issue described in
https://github.com/zephyrproject-rtos/zephyr/issues/11455



However, this did not fix the problem because the API is not
designed to be used in this way.  Finally commit 0e57e4fb
("samples: usb: cdc_acm: Update CDC ACM echo sample")
revised the sample to use the API in the correct way.

Remove incorrect comment in uart_fifo_fill(). For compatibility
reasons and due to the design of the API, such differences must not
exist.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>