Make pre_read imply invalidate=0

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index e22f745..494b3e6 100644
--- a/HOWTO
+++ b/HOWTO
@@ -714,7 +714,9 @@
 			when it's time to do IO to that file.
 
 pre_read=bool	If this is given, files will be pre-read into memory before
-		starting the given IO operation.
+		starting the given IO operation. This will also clear
+		the 'invalidate' flag, since it is pointless to pre-read
+		and then drop the cache.
 
 unlink=bool	Unlink the job files when done. Not the default, as repeated
 		runs of that job would then waste time recreating the file