add docs for alter sourrrrrrrrrrrrrrrrrrrrrrrrrrce

Signed-off-by: Bugen Zhao <i@bugenzhao.com>

src/frontend/src/handler/alter_source_with_sr.rs

@@ -81,6 +81,9 @@ fn encode_type_to_encode(from: EncodeType) -> Option<Encode> {
 /// - Hidden columns and `INCLUDE ... AS ...` columns are ignored. Because it's only for the special handling of alter sr.
 ///   For the newly resolved `columns_from_resolve_source` (created by [`bind_columns_from_source`]), it doesn't contain hidden columns (`_row_id`) and `INCLUDE ... AS ...` columns.
 ///   This is fragile and we should really refactor it later.
+/// - Column with the same name but different data type is considered as a different column, i.e., altering the data type of a column
+///   will be treated as dropping the old column and adding a new column. Note that we don't reject here like we do in `ALTER TABLE REFRESH SCHEMA`,
+///   because there's no data persistence (thus compatibility concern) in the source case.
 fn columns_minus(columns_a: &[ColumnCatalog], columns_b: &[ColumnCatalog]) -> Vec<ColumnCatalog> {
     columns_a
         .iter()