explain that ioflags can be used to give read-ahead hints to the underlying

filesystem.

share/man/man9/VOP_RDWR.9

@@ -60,7 +60,11 @@ the credentials of the caller
 .Pp
 The
 .Fa ioflag
-argument is a bit mask which can contain the following flags:
+argument is used to give directives and hints to the filesystem.
+When attempting a read, the high 16 bits are used to provide a
+read-ahead hint (in units of filesystem blocks) that the filesystem
+should attempt.  The low 16 bits are a bit mask which can contain
+the following flags:
 .Bl -tag -width IO_NODELOCKED
 .It Dv IO_UNIT
 do I/O as atomic unit

sys/sys/vnode.h

@@ -200,7 +200,7 @@ struct vattr {
 #define VA_EXCLUSIVE	0x02		/* exclusive create request */
 
 /*
- * Flags for ioflag.
+ * Flags for ioflag. (high 16 bits used to ask for read-ahead)
  */
 #define	IO_UNIT		0x01		/* do I/O as atomic unit */
 #define	IO_APPEND	0x02		/* append write to end */