added a new chapter for the netboot.

docs/user-ref.texi

@@ -150,6 +150,10 @@ standard interface used in all machines. However, some newer machines
 have the new interface, Logical Block Address (@dfn{LBA}) mode. GRUB
 automatically detects if LBA mode is available and uses it if
 available. In LBA mode, GRUB can access the entire disk.
+
+@item Network support
+GRUB is a disk-based boot loader but also has network support. You can
+load OS images from a network by using the @dfn{TFTP} protocol.
 @end table
 
 Future directions might include an internal programming language for
@@ -249,6 +253,10 @@ MBR when installing GRUB), while the syntax like @samp{(hd0,0)}
 represents using the partition of the disk (or the boot sector of the
 partition when installing GRUB).
 
+If you enabled the network support, the special drive, @samp{(nd)}, is
+also available. Before using the network drive, you must initiailze the
+network. @xref{Network}, for more information.
+
 
 @node Filename syntax
 @section How to specify files
@@ -371,6 +379,10 @@ cursor is after the first word, the @kbd{@key{TAB}} will provide a
 completion listing of disks, partitions, and filenames depending on the
 context.
 
+Note that you cannot use the completion functionality in the TFTP
+filesystem. This is because TFTP doesn't support filename listing for
+the security.
+
 
 @node Menu
 @section The simple menu interface