Specify the meaning of expression like: `"trial_type.congruent"`

[Remi-Gau]

Those appear in several examples (or examples in progress) in the model zoo and there is an in passing mention of it in the legacy spec but we should make it quite explicit that it means something along the line "accessing the specific 'rows' in a given column".

[Remi-Gau]

Related to this, is an implicit assumption that if a model only specifies trial_type in the X of a Model, it then implicitly assumes that only the rows that evaluate to "truthy" in the trial_type will be included.