[chore][GraphQL/Consistency] Update checkpoint_viewed_at comment

## Description

To reflect the fact that `checkpoint_viewed_at` is no longer an
optional parameter.

## Test plan

:eyes:

crates/sui-graphql-rpc/src/types/checkpoint.rs

@@ -319,8 +319,7 @@ impl Checkpoint {
             })
             .await?;
 
-        // Defer to the provided checkpoint_viewed_at, but if it is not provided, use the
-        // current available range. This sets a consistent upper bound for the nested queries.
+        // The "checkpoint viewed at" sets a consistent upper bound for the nested queries.
         let mut conn = Connection::new(prev, next);
         for stored in results {
             let cursor = stored.cursor(checkpoint_viewed_at).encode_cursor();

crates/sui-graphql-rpc/src/types/event.rs

@@ -222,8 +222,7 @@ impl Event {
 
         let mut conn = Connection::new(prev, next);
 
-        // Defer to the provided checkpoint_viewed_at, but if it is not provided, use the
-        // current available range. This sets a consistent upper bound for the nested queries.
+        // The "checkpoint viewed at" sets a consistent upper bound for the nested queries.
         for stored in results {
             let cursor = stored.cursor(checkpoint_viewed_at).encode_cursor();
             conn.edges.push(Edge::new(

crates/sui-graphql-rpc/src/types/transaction_block.rs

@@ -391,8 +391,7 @@ impl TransactionBlock {
 
         let mut conn = Connection::new(prev, next);
 
-        // Defer to the provided checkpoint_viewed_at, but if it is not provided, use the
-        // current available range. This sets a consistent upper bound for the nested queries.
+        // The "checkpoint viewed at" sets a consistent upper bound for the nested queries.
         for stored in results {
             let cursor = stored.cursor(checkpoint_viewed_at).encode_cursor();
             let inner = TransactionBlockInner::try_from(stored)?;