[adc_ctrl] Documentation of recommended ChromeOS detection ranges are insufficient

### Description

The documentation recommends how to program the `adc_chnX_filter_ctlN` registers, in order to detect voltage combinations as outlined in a diagram: https://opentitan.org/book/hw/ip/adc_ctrl/doc/programmers_guide.html

However, as far as I can tell, the listed values fail to take into account the error tolerance of the ADC itself (either +/- 30 mV or +/- 15 mV). That is, in order to e.g. recognize the voltage being in the range 400mV - 600mV, it is not good enough to calculate the ADC count corresponding to 400mV and to 600mV. We need instead to set the limits at 400mV - 15mV and 600mV + 30mV, to take into account the possible error introduced by the ADC itself.