add minor docs

Signed-off-by: Bugen Zhao <[email protected]>

src/common/src/hash/consistent_hash/mapping.rs

@@ -139,7 +139,10 @@ impl<T: VnodeMappingItem> VnodeMapping<T> {
         }
     }
 
-    /// Create a vnode mapping with the single item and length of 1. Only for singletons.
+    /// Create a vnode mapping with the single item and length of 1.
+    ///
+    /// Should only be used for singletons. If you want a different vnode count, call
+    /// [`VnodeMapping::new_uniform`] with `std::iter::once(item)` and desired length.
     pub fn new_single(item: T::Item) -> Self {
         Self::new_uniform(std::iter::once(item), 1)
     }

src/meta/model_v2/migration/src/m20240911_083152_variable_vnode_count.rs

@@ -21,6 +21,7 @@ impl MigrationTrait for Migration {
             )
             .await?;
 
+        // Fill vnode count with 1 for singleton tables.
         manager
             .exec_stmt(
                 UpdateStatement::new()
@@ -42,6 +43,7 @@ impl MigrationTrait for Migration {
             )
             .await?;
 
+        // Fill vnode count with 1 for singleton fragments.
         manager
             .exec_stmt(
                 UpdateStatement::new()