bad channel detection psd_hf_threshold units documentation #4745

The bad channel detection docs specify the units for psd_hf_threshold:

For coherence+psd - an absolute threshold (uV^2/Hz) used as a cutoff for noise channels. Channels with average power at >80% Nyquist larger than this threshold will be labeled as noise. IBL suggests 0.02 for AP data and 1.4 for LFP data.

This assumes that the data is in uV but this is at the discretion of the user. The docs can be changed to specify that it is in sample units with an example in the common uV case. Can double check how this is actually implemented under the hood too.