pgsql: Doc: clarify behavior of row-limit arguments in the PLs' SPI wra

Doc: clarify behavior of row-limit arguments in the PLs' SPI wrappers.

plperl, plpython, and pltcl all provide query-execution functions
that are thin wrappers around SPI_execute() or its variants.
The SPI functions document their row-count limit arguments clearly,
as "maximum number of rows to return, or 0 for no limit". However
the PLs' documentation failed to explain this special behavior of
zero, so that a reader might well assume it means "fetch zero
rows". Improve that.

Daniel Gustafsson and Tom Lane, per report from Kieran McCusker

Discussion: https://postgr.es/m/CAGgUQ6H6qYScctOhktQ9HLFDDoafBKHyUgJbZ6q_dOApnzNTXg@mail.gmail.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/825828956ab3c55bd22259d9c480b5f5d2d84416

Modified Files
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doc/src/sgml/plperl.sgml | 21 ++++++++++++++++-----
doc/src/sgml/plpython.sgml | 13 +++++++++++--
doc/src/sgml/pltcl.sgml | 8 +++++---
3 files changed, 32 insertions(+), 10 deletions(-)