usb: device_next: Clarify UAC2 send buffer requirements

The original intention was that the audio buffers supplied by
application to the UAC2 API can be directly used by UDC driver.
The buffer alignment requirement was explicitly stated with regards
to the receive buffer, but was mistakenly omitted on transmit buffer.

While the missing comment is added, mention also that the buffer has to
be otherwise suitable for use by UDC driver, because alignment might not
be the only UDC driver requirement. For example, the UDC driver can
require the buffer to reside in memory region that it can access with
DMA.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>