Update file_append documentation

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/HOWTO b/HOWTO
index 7e77c40..47eff96 100644
--- a/HOWTO
+++ b/HOWTO
@@ -438,7 +438,8 @@
 file_append=bool	Perform IO after the end of the file. Normally fio will
 		operate within the size of a file. If this option is set, then
 		fio will append to the file instead. This has identical
-		behavior to setting offset to the size of a file.
+		behavior to setting offset to the size of a file. This option
+		is ignored on non-regular files.
 
 fill_device=bool
 fill_fs=bool	Sets size to something really large and waits for ENOSPC (no