docs: Add a description to `TransactionRecorder` (#5875)

While one can look at the methods that `TransactionRecorder` provides
and infer what it does, it does not hurt to have a small note at the
type definition itself.  It shows up in the LSP hints and is immediately
visible when one looks at the type, without a need to look at the
implementation details.

poh/src/transaction_recorder.rs

@@ -40,6 +40,7 @@ pub struct RecordTransactionsSummary {
     pub starting_transaction_index: Option<usize>,
 }
 
+/// Adds transactions into the PoH stream, by sending them to the PoH service.
 #[derive(Clone, Debug)]
 pub struct TransactionRecorder {
     // shared by all users of PohRecorder