Fixed #30694 -- Documented FileResponse does not seek its file source.

docs/howto/outputting-pdf.txt

@@ -67,6 +67,7 @@ Here's a "Hello World" example::
 
         # FileResponse sets the Content-Disposition header so that browsers
         # present the option to save the file.
+        buffer.seek(0)
         return FileResponse(buffer, as_attachment=True, filename='hello.pdf')
 
 The code and comments should be self-explanatory, but a few things deserve a

docs/ref/request-response.txt

@@ -1122,6 +1122,8 @@ Attributes
 
     If ``open_file`` doesn't have a name or if the name of ``open_file`` isn't
     appropriate, provide a custom file name using the ``filename``  parameter.
+    Note that if you pass a file-like object like ``io.BytesIO``, it's your
+    task to ``seek()`` it before passing it to ``FileResponse``.
 
     The ``Content-Length`` and ``Content-Type`` headers are automatically set
     when they can be guessed from contents of ``open_file``.