git-fast-import.txt: fix documentation for --quiet option

Signed-off-by: Elijah Newren <newren@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fast-import.txt

@@ -40,9 +40,10 @@ OPTIONS
 	not contain the old commit).
 
 --quiet::
-	Disable all non-fatal output, making fast-import silent when it
-	is successful.  This option disables the output shown by
-	--stats.
+	Disable the output shown by --stats, making fast-import usually
+	be silent when it is successful.  However, if the import stream
+	has directives intended to show user output (e.g. `progress`
+	directives), the corresponding messages will still be shown.
 
 --stats::
 	Display some basic statistics about the objects fast-import has