spi: docs: update info about 'delay_usecs'

        duplex (one pointer is NULL) transfers;

      + optionally defining short delays after transfers ... using

        the spi_transfer.delay_usecs setting (this delay can be the

        only protocol effect, if the buffer length is zero);

        the spi_transfer.delay.value setting (this delay can be the

        only protocol effect, if the buffer length is zero) ...

        when specifying this delay the default spi_transfer.delay.unit

        is microseconds, however this can be adjusted to clock cycles

        or nanoseconds if needed;

      + whether the chipselect becomes inactive after a transfer and

        any delay ... by using the spi_transfer.cs_change flag;