Add comment to read op semantics for direct io

fuseops/ops.go

@@ -600,6 +600,9 @@ type ReadFileOp struct {
 	// (http://goo.gl/SGxnaN) to read a page at a time. It appears to understand
 	// where EOF is by checking the inode size (http://goo.gl/0BkqKD), returned
 	// by a previous call to LookUpInode, GetInodeAttributes, etc.
+	//
+	// If direct io is enabled, semantics should match those of read (man 2
+	// read).
 	BytesRead int
 }