pgsql: Doc: update libpq's documentation for PQfn().

Doc: update libpq's documentation for PQfn().

Mention specifically that you can't call aggregates, window functions,
or procedures this way (the inability to call SRFs was already
mentioned).

Also, the claim that PQfn doesn't support NULL arguments or results
has been a lie since we invented protocol 3.0.  Not sure why this
text was never updated for that, but do it now.

Discussion: https://postgr.es/m/2039442.1615317309@sss.pgh.pa.us

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2033d108ee6b7328a17175645bc86c127279bca0

Modified Files
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doc/src/sgml/libpq.sgml | 26 +++++++++++++++++++++++---
1 file changed, 23 insertions(+), 3 deletions(-)