Document hb_feature_t.

This documents hb_feature_t. This is motivated mostly by the ambiguity
of the units for 'start' and 'end' (clusters) and whether they are
inclusive or exclusive. This also documents that for lookup type 3 the
value is the one based index into the alternates and that in a list of
features later feature values override previous feature values with the
same tag.
2 files changed