docs: qsd: Explain --export nbd,name=... default

The 'name' option for NBD exports is optional. Add a note that the
default for the option is the node name (people could otherwise expect
that it's the empty string like for qemu-nbd).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210305094856.18964-1-kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

docs/tools/qemu-storage-daemon.rst

@@ -80,8 +80,9 @@ Standard options:
   requests for modifying data (the default is off).
 
   The ``nbd`` export type requires ``--nbd-server`` (see below). ``name`` is
-  the NBD export name. ``bitmap`` is the name of a dirty bitmap reachable from
-  the block node, so the NBD client can use NBD_OPT_SET_META_CONTEXT with the
+  the NBD export name (if not specified, it defaults to the given
+  ``node-name``). ``bitmap`` is the name of a dirty bitmap reachable from the
+  block node, so the NBD client can use NBD_OPT_SET_META_CONTEXT with the
   metadata context name "qemu:dirty-bitmap:BITMAP" to inspect the bitmap.
 
   The ``vhost-user-blk`` export type takes a vhost-user socket address on which