chore: Update Blockstore::get_slots_since() doc comments (#31134)

Additionally, change the function definition to use Slot instead of u64.
Slot is defined as an alias of u64 so these are functionally equivalent,
but using Slot over u64 is more expressive of the intent.

ledger/src/blockstore.rs

@@ -3034,13 +3034,14 @@ impl Blockstore {
         entries.into_iter().flatten().collect()
     }
 
-    // Returns slots connecting to any element of the list `slots`.
-    pub fn get_slots_since(&self, slots: &[u64]) -> Result<HashMap<u64, Vec<u64>>> {
+    /// Returns a mapping from each elements of `slots` to a list of the
+    /// element's children slots.
+    pub fn get_slots_since(&self, slots: &[Slot]) -> Result<HashMap<Slot, Vec<Slot>>> {
         let slot_metas: Result<Vec<Option<SlotMeta>>> =
             self.meta_cf.multi_get(slots.to_vec()).into_iter().collect();
         let slot_metas = slot_metas?;
 
-        let result: HashMap<u64, Vec<u64>> = slots
+        let result: HashMap<Slot, Vec<Slot>> = slots
             .iter()
             .zip(slot_metas)
             .filter_map(|(slot, meta)| meta.map(|meta| (*slot, meta.next_slots.to_vec())))