Fix javadoc for LongCombiner #2047
Description
The javadoc for LongCombiner has some inaccurate information about the supported types on lines 48-49. Based on the Type enum, the actual supported types are VARLEN, FIXEDLEN, and STRING, which are provided by the VarLenEncoder, FixedLenEncoder, and StringEncoder. The javadoc should be updated to describe the correct types.