Instead of not saying anything when we have no documentation, say "Not
documented".

Example change (qemu-qmp-ref.txt):

  -- Enum: GuestPanicAction

      An enumeration of the actions taken when guest OS panic is detected

      Values:
      'pause'
           system pauses
      'poweroff'
+          Not documented

      Since: 2.1 (poweroff since 2.8)
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-23-git-send-email-armbru@redhat.com>