Improve code comments for ledgerstore columns. (#28054)

### Problem
The documentation of each column family is missing

### Summary
The goal is to create a comment block that will essentially include a high-level
concept on what each column family is about and what are their key/value formats.

This PR is the first cut that includes the key/value format of each column family.
This should at least provide an easy pointer for readers to understand what this
column family stores by searching its value type and how to access the data based
on the key type.

ledger/src/blockstore_db.rs

@@ -135,82 +135,142 @@ pub enum IteratorMode<Index> {
 pub mod columns {
     #[derive(Debug)]
     /// The slot metadata column
+    ///
+    /// index type: u64 (see `SlotColumn`)
+    /// value type: `blockstore_meta::SlotMeta`
     pub struct SlotMeta;
 
     #[derive(Debug)]
     /// The orphans column
+    ///
+    /// index type: u64 (see `SlotColumn`)
+    /// value type: bool
     pub struct Orphans;
 
     #[derive(Debug)]
     /// The dead slots column
+    ///
+    /// index type: u64 (see `SlotColumn`)
+    /// value type: bool
     pub struct DeadSlots;
 
     #[derive(Debug)]
     /// The duplicate slots column
+    ///
+    /// index type: u64 (see `SlotColumn`)
+    /// value type: `blockstore_meta::DuplicateSlotProof`
     pub struct DuplicateSlots;
 
     #[derive(Debug)]
     /// The erasure meta column
+    ///
+    /// index type: (u64, u64)
+    /// value type: `blockstore_meta::ErasureMeta`
     pub struct ErasureMeta;
 
     #[derive(Debug)]
     /// The bank hash column
+    ///
+    /// index type: u64 (see `SlotColumn`)
+    /// value type: `blockstore_meta::FrozenHashVersioned`
     pub struct BankHash;
 
     #[derive(Debug)]
     /// The root column
+    ///
+    /// index type: u64 (see `SlotColumn`)
+    /// value type: bool
     pub struct Root;
 
     #[derive(Debug)]
     /// The index column
+    ///
+    /// index type: u64 (see `SlotColumn`)
+    /// value type: `blockstore_meta::Index`
     pub struct Index;
 
     #[derive(Debug)]
     /// The shred data column
+    ///
+    /// index type: (u64, u64)
+    /// value type: Vec<u8>
     pub struct ShredData;
 
     #[derive(Debug)]
     /// The shred erasure code column
+    ///
+    /// index type: (u64, u64)
+    /// value type: Vec<u8>
     pub struct ShredCode;
 
     #[derive(Debug)]
     /// The transaction status column
+    ///
+    /// index type: (u64, Signature, Slot)
+    /// value type: `generated::TransactionStatusMeta`
     pub struct TransactionStatus;
 
     #[derive(Debug)]
     /// The address signatures column
+    ///
+    /// index type: (u64, Pubkey, Slot, Signature)
+    /// value type: `blockstore_meta::AddressSignatureMeta`
     pub struct AddressSignatures;
 
     #[derive(Debug)]
     /// The transaction memos column
+    ///
+    /// index type: Signature
+    /// value type: String
     pub struct TransactionMemos;
 
     #[derive(Debug)]
-    /// The transaction status index column
+    /// The transaction status index column.
+    ///
+    /// index type: u64 (see `SlotColumn`)
+    /// value type: `blockstore_meta::TransactionStatusIndexMeta`
     pub struct TransactionStatusIndex;
 
     #[derive(Debug)]
     /// The rewards column
+    ///
+    /// index type: u64 (see `SlotColumn`)
+    /// value type: `generated::Rewards`
     pub struct Rewards;
 
     #[derive(Debug)]
     /// The blocktime column
+    ///
+    /// index type: u64 (see `SlotColumn`)
+    /// value type: `UnixTimestamp`
     pub struct Blocktime;
 
     #[derive(Debug)]
     /// The performance samples column
+    ///
+    /// index type: u64 (see `SlotColumn`)
+    /// value type: `blockstore_meta::PerfSample`
     pub struct PerfSamples;
 
     #[derive(Debug)]
     /// The block height column
+    ///
+    /// index type: u64 (see `SlotColumn`)
+    /// value type: u64
     pub struct BlockHeight;
 
     #[derive(Debug)]
     /// The program costs column
+    ///
+    /// index type: `Pubkey`
+    /// value type: `blockstore_meta::ProgramCost`
     pub struct ProgramCosts;
 
     #[derive(Debug)]
     /// The optimistic slot column
+    ///
+    /// index type: u64 (see `SlotColumn`)
+    /// value type: `blockstore_meta::OptimisticSlotMetaVersioned`
     pub struct OptimisticSlots;
 
     // When adding a new column ...
@@ -541,26 +601,32 @@ pub trait ProtobufColumn: Column {
     type Type: prost::Message + Default;
 }
 
+/// SlotColumn is a trait for slot-based column families.  Its index is
+/// essentially Slot (or more generally speaking, has a 1:1 mapping to Slot).
 pub trait SlotColumn<Index = u64> {}
 
 impl<T: SlotColumn> Column for T {
     type Index = u64;
 
+    /// Converts a u64 Index to its RocksDB key.
     fn key(slot: u64) -> Vec<u8> {
         let mut key = vec![0; 8];
         BigEndian::write_u64(&mut key[..], slot);
         key
     }
 
+    /// Converts a RocksDB key to its u64 Index.
     fn index(key: &[u8]) -> u64 {
         BigEndian::read_u64(&key[..8])
     }
 
+    /// Obtains the primary index from the specified index.
     fn primary_index(index: u64) -> Slot {
         index
     }
 
     #[allow(clippy::wrong_self_convention)]
+    /// Converts a Slot to its u64 Index.
     fn as_index(slot: Slot) -> u64 {
         slot
     }