move the description of the *sessions.tsv file in the Modality agnostic files section

[Remi-Gau]

Currently it is "hidden" in the longitudinal section which does not help users to find it unless they know where to look for.

https://bids-specification.readthedocs.io/en/stable/06-longitudinal-and-multi-site-studies.html

[sappelhoff]

agreed!

[effigies]

Sounds good. Possibly we could expand this section to include other uses for session, such as hyperscanning dyadics.

[Remi-Gau]

such as hyperscanning dyadics.

it is almost as if someone posted a question about that recently. :-p