docs: ntb.txt: add blank lines to clean up some Sphinx warnings

As one can see the described scenarios can be combined in one portable

algorithm.

 Local device:

  1) Allocate memory for a shared window

  2) Initialize memory window by translated address of the allocated region

     (it may fail if local memory window initialization is unsupported)

  3) Send the translated address and memory window index to a peer device

 Peer device:

  1) Initialize memory window with retrieved address of the allocated

     by another device memory region (it may fail if peer memory window

In accordance with this scenario, the NTB Memory Window API can be used as

follows:

 Local device:

  1) ntb_mw_count(pidx) - retrieve number of memory ranges, which can

     be allocated for memory windows between local device and peer device

  5) Send translated base address (usually together with memory window

     number) to the peer device using, for instance, scratchpad or message

     registers.

 Peer device:

  1) ntb_peer_mw_set_trans(pidx, midx) - try to set received from other

     device (related to pidx) translated address for specified memory