clarify usage of the updated statement handle

format/FlightSql.proto

@@ -1797,15 +1797,24 @@ message DoPutUpdateResult {
   int64 record_count = 1;
 }
 
-// An optional response returned when `DoPut` is called with `CommandPreparedStatementQuery`
+// An *optional* response returned when `DoPut` is called with `CommandPreparedStatementQuery`.
+//
+// *Note on legacy behavior*: previous versions of the protocol did not return any result for
+// this command, and that behavior should still be supported by clients. See documentation
+// of individual fields for more details on expected client behavior in this case. 
 message DoPutPreparedStatementResult {
   option (experimental) = true;
 
-  // (potentially updated) opaque handle for the prepared statement on the server.
-  // All subsequent requests for this prepared statement must use this new handle, if specified.
+  // Represents a (potentially updated) opaque handle for the prepared statement on the server.
+  // Because the handle could potentially be updated, any previous handles for this prepared
+  // statement should be considered invalid, and all subsequent requests for this prepared
+  // statement must use this new handle, if specified.
   // The updated handle allows implementing query parameters with stateless services
-  // as described in https://github.com/apache/arrow/issues/37720
-  bytes prepared_statement_handle = 1;
+  // as described in https://github.com/apache/arrow/issues/37720.
+  // 
+  // When an updated handle is not provided by the server, clients should contiue
+  // using the previous handle provided by `ActionCreatePreparedStatementResonse`.
+  optional bytes prepared_statement_handle = 1;
 }
 
 /*