descriptions.tsv not fully supported in schema

[tsalo]

There are two problems I wasn't able to address in #1707:

1. The filename template-building macro isn't working. This is because the descriptions.tsv file has unique rules, unlike other files in the schema. Namely, it can appear at the top level of the dataset, or in subject-level folders, or in session-level folders, but not in datatype-level folders.
   • I think trying to support subject-level and session-level descriptions is probably unnecessary, and overcomplicates things.
2. Rules/checks not implemented in schema.
   • We probably want to warn users if any descriptions in the TSV file aren't reflected in the dataset, or vice versa, but I couldn't figure out how to actually implement this in the schema, nor in the validator.

Originally posted by @tsalo in #1707 (comment)