Fix example of JSON export with flatc (#8892)

Binary files should be placed after "--". Also add a note about missing
file_identifier and --raw-binary.

docs/source/flatc.md

diff --git a/docs/source/flatc.md b/docs/source/flatc.md
index 330ebc3..598992e 100644
--- a/docs/source/flatc.md
+++ b/docs/source/flatc.md
@@ -79,11 +79,12 @@
     `myschema.fbs` to JSON:
 
     ```sh
-    flatc --json myschema.fbs mydata.bin
+    flatc --json myschema.fbs -- mydata.bin
     ```
 
-    This will generate a `mydata.json` file.
-
+    This will generate a `mydata.json` file. If there is no
+    [`file_identifier`](schema.md/#file-identification-and-extension) defined
+    for this schema, you will need to use the `--raw-binary` option.
 
 ### Additional options