GH-541: Document status of file_path (#542)

Author: emkornfield

src/main/thrift/parquet.thrift

@@ -963,6 +963,21 @@ union ColumnCryptoMetaData {
 struct ColumnChunk {
   /** File where column data is stored.  If not set, assumed to be same file as
     * metadata.  This path is relative to the current file.
+    *
+    * As of December 2025, the only known use-case for this field is writing summary 
+    * parquet files (i.e. "_metadata" files).  These files consolidate footers from 
+    * multiple parquet files to allow for efficient reading of footers to avoid file 
+    * listing costs and prune out files that do not need to be read based on statistics.
+    *
+    * These files do not appear to have ever been formally specified in the specification.
+    * and are potentially problematic from a correctness perspective [1].
+    * 
+    * [1] https://lists.apache.org/thread/ootf2kmyg3p01b1bvplpvp4ftd1bt72d
+    *
+    * There is no other known usage of this field. Specifically, there are no known 
+    * reference implementations that will read externally stored column data if this field is populated 
+    * within a standard parquet file. Making use of the field for this purpose is  
+    * not considered part of the Parquet specification.
     **/
   1: optional string file_path