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

 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