NSEdoc cleanup

Mostly splitting function summaries (the first paragraph of NSEdoc) from
the body of the description to make the summary indexes shorter and
easier to scan.

Also fixed some unbalanced code tags like <code>foo</table>

nselib/tab.lua

@@ -71,9 +71,10 @@ function addrow(t, ...)
   nextrow(t)
 end
 
---- Move on to the next row in the table. If this is not called
--- then previous column values will be over-written by subsequent
--- values.
+--- Move on to the next row in the table.
+--
+-- If this is not called then previous column values will be over-written by
+-- subsequent values.
 -- @param t The table.
 function nextrow(t)
   assert(t)