add documentation for new callback-based functions
add documentation for new callback-based functions

waitman/flac,kode54/flac,LordJZ/libflac,Distrotech/flac,Distrotech/flac,LordJZ/libflac,fredericgermain/flac,kode54/flac,waitman/flac,kode54/flac,LordJZ/libflac,fredericgermain/flac,fredericgermain/flac,Distrotech/flac,waitman/flac,waitman/flac,fredericgermain/flac,waitman/flac,Distrotech/flac,LordJZ/libflac,kode54/flac