Documentation: nbd: Reformat to allow more documentation

Reformat the existing documentation to have more structure. This allows
for more documentation seperated from the existing paragraphs.
Signed-off-by: NMarkus Pargmann <mpa@pengutronix.de>
Signed-off-by: NJens Axboe <axboe@fb.com>

Documentation/blockdev/nbd.txt

-                      Network Block Device (TCP version)
-                                       
-   What is it: With this compiled in the kernel (or as a module), Linux
-   can use a remote server as one of its block devices. So every time
-   the client computer wants to read, e.g., /dev/nb0, it sends a
-   request over TCP to the server, which will reply with the data read.
-   This can be used for stations with low disk space (or even diskless)
-   to borrow disk space from another computer.
-   Unlike NFS, it is possible to put any filesystem on it, etc.
+Network Block Device (TCP version)
+==================================
 
-   For more information, or to download the nbd-client and nbd-server
-   tools, go to http://nbd.sf.net/.
+1) Overview
+-----------
 
-   The nbd kernel module need only be installed on the client
-   system, as the nbd-server is completely in userspace. In fact,
-   the nbd-server has been successfully ported to other operating
-   systems, including Windows.
+What is it: With this compiled in the kernel (or as a module), Linux
+can use a remote server as one of its block devices. So every time
+the client computer wants to read, e.g., /dev/nb0, it sends a
+request over TCP to the server, which will reply with the data read.
+This can be used for stations with low disk space (or even diskless)
+to borrow disk space from another computer.
+Unlike NFS, it is possible to put any filesystem on it, etc.
+
+For more information, or to download the nbd-client and nbd-server
+tools, go to http://nbd.sf.net/.
+
+The nbd kernel module need only be installed on the client
+system, as the nbd-server is completely in userspace. In fact,
+the nbd-server has been successfully ported to other operating
+systems, including Windows.