Documentation update for PQexecParams

Hi

I have been using PQprepare() and PQexecPrepared(). This has led me to
update libpq.sgml, in particular for PQexecParams().

The changes are to the formatting of the arguments for PQexecParams()
(as shared by PQexecPrepared()) which make it easier to read, as well as
creating a table giving a set of possible values for the various arguments with
the associated SQL types.

I would imagine the best place to submit the patch would be to
psql-patches, however, before I do this I am wondering if this is the right forum for
having the changes vetted before submitting the patch.

Please advise.
--
Regards
Theo