add more docs

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

src/frontend/src/catalog/table_catalog.rs

@@ -174,15 +174,14 @@ pub struct TableCatalog {
 
     /// Total vnode count of the table.
     ///
-    /// Can be unset if the catalog is generated by the frontend and not yet persisted. This is
+    /// Can be placeholder if the catalog is generated by the frontend and not yet persisted. This is
     /// because the vnode count of each fragment (then internal tables) is determined by the
-    /// meta service. See also [`StreamMaterialize::derive_table_catalog`] and
-    /// [`TableCatalogBuilder::build`].
+    /// meta service. In this case, calling [`TableCatalog::vnode_count()`] will panic.
+    /// See also [`StreamMaterialize::derive_table_catalog`] and [`TableCatalogBuilder::build`].
     ///
-    /// On the contrary, if this comes from a [`PbTable`], the field must be `Some` no matter
+    /// On the contrary, if this comes from a [`PbTable`], the field must have value no matter
     /// whether the table is created before or after the version we introduced variable vnode
-    /// count support. This is because we've already handled backward compatibility during
-    /// conversion.
+    /// count support. We will handle backward compatibility when obtaining the value.
     ///
     /// [`StreamMaterialize::derive_table_catalog`]: crate::optimizer::plan_node::StreamMaterialize::derive_table_catalog
     /// [`TableCatalogBuilder::build`]: crate::optimizer::plan_node::utils::TableCatalogBuilder::build

src/meta/model_v2/src/table.rs

@@ -211,7 +211,7 @@ impl From<PbTable> for ActiveModel {
         let handle_pk_conflict_behavior = pb_table.handle_pk_conflict_behavior();
 
         // `PbTable` here should be sourced from the wire, not from persistence.
-        // An unset `maybe_vnode_count` field should be treated as `NotSet`, instead of calling
+        // A placeholder `maybe_vnode_count` field should be treated as `NotSet`, instead of calling
         // the compatibility code.
         let vnode_count = pb_table
             .vnode_count_inner()