Updating the specification of .tsv in common principles

[VisLab]

This issue is being separated from the main part of issue #537: The current specification of tabular-files is contradictory. It needs REQUIRED/RECOMMENDED/OPTIONAL clarification as per discussion between @VisLab and @sappelhoff:

ust to link: https://bids-specification.readthedocs.io/en/stable/02-common-principles.html#tabular-files

What is the required relationship between this key and the columns of the spreadsheet and how does it relate to "LongName".

the key is what is in the TSV, LongName is the unabbreviated name of the column ... for example bmi as the key, Body Mass Index as the LongName

If so, I think that the explanation for LongName in the table should be clarified.

Clarifications are always welcome in the form of a Pull Request 👍

The spec should say what to do since the description above appears to say that all of these are required.

Indeed, that'd be another welcome clarification. As it reads right now it's really unclear --> but for a fact, it's impossible to REQUIRE all of the fields, because a column in which you can define Units CANNOT logically have Levels.

I think the only REQUIRED field should be Description, the others should be RECOMMENDED/OPTIONAL.