Update flatc.md (#8821)

docs/source/flatc.md

diff --git a/docs/source/flatc.md b/docs/source/flatc.md
index be6d8f1..330ebc3 100644
--- a/docs/source/flatc.md
+++ b/docs/source/flatc.md
@@ -261,6 +261,9 @@
 
 -   `--python-decode-obj-api-strings` : Decode bytes automaticaly with utf-8
 
+-   `--file-names-only` : Prints out files which would be generated by this command, one per
+    line, to `stdout`. No actual files are generated. This is useful for various CI checks.
+
 Additional gRPC options:
 
 -   `--grpc-filename-suffix`: `[C++]` An optional suffix for the generated