Improve SlotColumn's code comment. (solana-labs#28447)

Improve SlotColumn's code comment to include how
LedgerCleanupService manages the clean-up of a SlotColumn.

ledger/src/blockstore_db.rs

@@ -627,6 +627,11 @@ pub trait ProtobufColumn: Column {
 
 /// 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).
+///
+/// The clean-up of any LedgerColumn that implements SlotColumn is managed by
+/// [`LedgerCleanupService`], which will periodically deprecate and purge
+/// oldest entries that are older than the latest root in order to maintain the
+/// configured --limit-ledger-size under the validator argument.
 pub trait SlotColumn<Index = u64> {}
 
 impl<T: SlotColumn> Column for T {