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;