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