[FIX] Clarify language on unsetting a key/value pair

[nicholst]

closes #492 where @Lestropie observed that the current language on inheritance is very confusing.

This small PR simply replaces this (common principles) text

There is no notion of "unsetting" a key/value pair. For example if there is a JSON file corresponding to particular participant/run defining a key/value and there is a JSON file on the root level of the dataset that does not define this key/value it will not be "unset" for all subjects/runs.

with

There is no notion of "unsetting" a key/value pair. Once a key/value pair is set in a given level in the dataset, lower down in the hierarchy that key/value pair will always have some assigned value.

[effigies]

LGTM.