Fix documentation of hb_subset_flags_t defaults HB_SUBSET_FLAGS_DEFAULT is 0, and hb_subset_input_create_or_fail initializes its flags to HB_SUBSET_FLAGS_DEFAULT, so currently all flags are set to false by default. It has been decided that future flags will also be false by default for simplicity (using NO_ in the name if needed). Correct and simplify the documentation for hb_subset_flags_t to reflect this.