DOC: add warning to save DigMontage: ch_names are not saved

[sappelhoff]

See title. I think we should have a save method for DigMontage that allows for an actual roundtrip.

[larsoner]

Rather than this I think we should pursue #12994 (comment) and add a .. versionchanged:: to note that in 1.9+ they are saved

[sappelhoff]

Rather than this I think we should pursue #12994 (comment) and add a .. versionchanged:: to note that in 1.9+ they are saved

I agree in general, however I also think it doesn't hurt to merge this one, while we wait for the better implementation.

In addition to writing channel names, we could perhaps in also start recommending to use *-dig.fif[.gz] as the filename. AND we would need to adjust the reading code in https://mne.tools/stable/generated/mne.channels.read_dig_fif.html#mne.channels.read_dig_fif

[larsoner]

I could probably push the reading changes quickly tomorrow if you want, unless you wanted to work on it?

[sappelhoff]

I could probably push the reading changes quickly tomorrow if you want, unless you wanted to work on it?

nope, if you have some time to do it and replace the warning here with something ACTUALLY useful, than that'd make me super happy!

I have been doing a few small PRs here and there whenever I see something that I can fix in a few minutes, but my priority list currently doesn't really allow me to dig deeper for most of these items.