[TypeInfo] Improve Type documentation
Better distinguish between file formats which use an encoding
(are "sub-class-of" in shared-mime-info nomenclature) as an
implementation detail; and where the mime-type directly maps to a user
facing Type
category.
Text
is actually the outlier here, as it typically is not a Type which
provides any information for the user. Notably, text/plain
is both
Text
and Document
.