trivial doc fixes

[John W. Eaton]

On 29-Oct-2008, Søren Hauberg wrote:

| Hi,
|   The manual currently contain @DOCSTRING's that reference the following
| non-existing functions:
| 
|   swap, swapcols, swaprows, gnuplot_use_title_option, document
| 
| Also, I think the following substitutions of @DOCSTRING's are in order
| 
|   backends --> backend
|   validstring --> validatestring
| 
| And just to make the patch a bit larger, it seems 'polyvalm' has an
| 'and' in its @seealso list. This should simple be removed, as the
| convention is to only list functions in @seealso's.
| 
| Patch attached,

I applied it.

Thanks,

jwe