Add warning about possible concurrent tile race #1151

docs/api-output.md

@@ -279,6 +279,8 @@ Use tile-based deep zoom (image pyramid) output.
 Set the format and options for tile images via the `toFormat`, `jpeg`, `png` or `webp` functions.
 Use a `.zip` or `.szi` file extension with `toFile` to write to a compressed archive file format.
 
+Warning: multiple sharp instances concurrently producing tile output can expose a possible race condition in some versions of libgsf.
+
 **Parameters**
 
 -   `tile` **[Object][15]?**