Make it clearer that users may pass a `DigMontage` as an EEG template montage

[hoechenberger]

https://mne.discourse.group/t/custom-montage-with-mne-bids-pipeline/11389/2?u=richard

Background is that currently the docs show our custom type, DigMontageType, and don't expand it, so users don't know what's actually behind it. We should reconsider this. Or at least document the type itself