FIX: Adds more documentation

base/src/test/mod.rs

@@ -47,6 +47,7 @@ mod test_fn_offset;
 mod test_number_format;
 
 mod test_escape_quotes;
+mod test_extend;
 mod test_fn_type;
 mod test_frozen_rows_and_columns;
 mod test_get_cell_content;