code: func (idx *CfIndex) FilterHashesByBlockHashes(blockHashes []*chainhash.Hash,
  	filterType wire.FilterType) ([][]byte, error) {
  	return idx.entriesByBlockHashes(cfHashKeys, filterType, blockHashes)
  }
docs: // FilterHashesByBlockHashes returns the serialized contents of a block's basic
  // committed filter hash for a set of blocks by hash.