[FEATURE] Please clarify what triggers "match_key" in YAML configurations

[zstadler]

The documentation for match_key in YAML configurations says:

• match_key - string tag that triggered a match to include the feature in this layer

It is not clear to me which tag is selected when include_when is a complex boolean expression, involving nested __all__, __any__, and __not__.

Perhaps the documentation should say:

• match_key - string tag that triggered a match to include the feature in this layer. In complex boolean expression this would be the first tag in reading order of the expression, that matches the element, ignoring all __not__ sub-expressions.