Update documentation for filename/directory

Sync the HOWTO and man page, and add a reference from 'directory'
to 'filename' on how to escape certain characters.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/HOWTO b/HOWTO
index 9a9f02a..725214f 100644
--- a/HOWTO
+++ b/HOWTO
@@ -262,7 +262,8 @@
 		not parsed.
 
 directory=str	Prefix filenames with this directory. Used to place files
-		in a different location than "./".
+		in a different location than "./". See the 'filename' option
+		for escaping certain characters.
 
 filename=str	Fio normally makes up a filename based on the job name,
 		thread number, and file number. If you want to share