Add documentation about duplicated metadata in Electrodes

[h-mayorquin]

Motivation

As requested by @bendichter there are thee parts to this documentation.

• A general comment general about duplicating metadata.
• A general comment in the ecephys section of Electrodes about not writing columns that might be somewhere else.
• A very specific comment about avoiding writing the Spikeinterface's properties offset_to_uv and gain_to_uv to Electrodes.

Once we are satisfied with the documentation I can add a specific check for the spikeinterface mistake if that is desired.