Clarifying anonymization for EDF files for EEG/iEEG

[adam2392]

Issue raised in mne-tools/mne-bids#667

Summary

Anonymization in BIDS recommends a date < 1925. However, in EDF file format, the minimum date allowed is 1985.01.01 00:00:00.

In mne-bids discussion, we proposed that a BIDS compliant anonymization of an EDF file would consist of:

1. setting EDF recording date to the minimum.
2. having scans.tsv acq_time be the source of truth for the "anonymized" recording date (i.e. 1924.01.01 )

Proposed Fix

I want to propose adding a PR to clarify this in the scans.tsv section around recording date anonymization and add a note for EDF files.

[sappelhoff]

related: #538

[sappelhoff]

#761 would play well with this clarification. Because we would have a mismatch of BIDS metadata and EDF metadata, and BIDS would just overwrite EDF.