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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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8e9b7c63877094bee343283d00b4e29bd9bb4ffa

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