Add doc for TableCommit (#1263)

## Which issue does this PR close?

- Closes #1262 .

## What changes are included in this PR?

Update doc.

## Are these changes tested?

No.

Author: liurenjie1024

crates/iceberg/src/catalog/mod.rs

@@ -268,6 +268,10 @@ pub struct TableCreation {
 }
 
 /// TableCommit represents the commit of a table in the catalog.
+///
+/// The builder is marked as private since it's dangerous and error-prone to construct
+/// [`TableCommit`] directly.
+/// Users are supposed to use [`crate::transaction::Transaction`] to update table.
 #[derive(Debug, TypedBuilder)]
 #[builder(build_method(vis = "pub(crate)"))]
 pub struct TableCommit {