improve PQexec documentation

Hello devs,

I'm looking at psql's use of PQexec for implementing some feature.

When running with multiple SQL commands, the doc is not very helpful.

From the source code I gathered that PQexec returns the first COPY results
if any, and if not the last non-empty results, unless all is empty in
which case an empty result is returned. So * marks the returned result
in the following examples:

INSERT ... \; * COPY ... \; SELECT ... \; \;
SELECT ... \; UPDATE ... \; * SELECT ... \; \;
\; \; * ;

The attached patch tries to improve the documentation based on my
understanding.

IMVHO, psql's code is kind of a mess to work around this strange behavior,
as there is a loop over results within PQexec, then another one after
PQexec if there were some COPY.

--
Fabien.