Add comment block for ErasureMeta ledger column (#28356)

Add comment block for ErasureMeta ledger column.

ledger/src/blockstore_db.rs

@@ -166,9 +166,16 @@ pub mod columns {
  pub struct DuplicateSlots;
 
  #[derive(Debug)]
- /// The erasure meta column
+ /// The erasure meta column.
  ///
- /// index type: (u64, u64)
+ /// This column family stores ErasureMeta which includes metadata about
+ /// dropped network packets (or erasures) that can be used to recover
+ /// missing data shreds.
+ ///
+ /// Its index type is ErasureSetId, which consists of a Slot ID
+ /// and a FEC (Forward Error Correction) set index.
+ ///
+ /// index type: `ErasureSetId` (Slot, fec_set_index: u64)
  /// value type: `blockstore_meta::ErasureMeta`
  pub struct ErasureMeta;