doc: add_parquet_files is not fully supported for version 0.5.0 (#1187)

## Which issue does this PR close?

- Mentioned in #1035

Co-authored-by: Renjie Liu <[email protected]>

Author: jonathanc-n

crates/iceberg/src/transaction/append.rs

@@ -73,6 +73,11 @@ impl<'a> FastAppendAction<'a> {
     }
 
     /// Adds existing parquet files
+    ///
+    /// Note: This API is not yet fully supported in version 0.5.0.  
+    /// It is currently incomplete and should not be used in production.
+    /// Specifically, schema compatibility checks and support for adding to partitioned tables
+    /// have not yet been implemented.
     #[allow(dead_code)]
     async fn add_parquet_files(mut self, file_path: Vec<String>) -> Result<Transaction<'a>> {
         if !self