Add fill_fs alias for fill_device

Name implies that it works on a device, which isn't quite correct.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/fio.1 b/fio.1
index a6547ee..a8c0027 100644
--- a/fio.1
+++ b/fio.1
@@ -231,7 +231,7 @@
 full size of the given files or devices. If the the files do not exist, size
 must be given.
 .TP
-.BI fill_device \fR=\fPbool
+.BI fill_device \fR=\fPbool "\fR,\fB fill_fs" \fR=\fPbool
 Sets size to something really large and waits for ENOSPC (no space left on
 device) as the terminating condition. Only makes sense with sequential write.
 For a read workload, the mount point will be filled first then IO started on