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

doc/src/sgml/plperl.sgml

@@ -441,7 +441,7 @@ use strict;
    <variablelist>
     <varlistentry>
      <term>
-      <literal><function>spi_exec_query</function>(<replaceable>query</replaceable> [, <replaceable>max-rows</replaceable>])</literal>
+      <literal><function>spi_exec_query</function>(<replaceable>query</replaceable> [, <replaceable>limit</replaceable>])</literal>
       <indexterm>
        <primary>spi_exec_query</primary>
        <secondary>in PL/Perl</secondary>
@@ -449,9 +449,17 @@ use strict;
      </term>
      <listitem>
       <para>
-       <literal>spi_exec_query</literal> executes an SQL command and
+       <function>spi_exec_query</function> executes an SQL command and
-returns the entire row set as a reference to an array of hash
+returns the entire row set as a reference to an array of hash references.
-references.  <emphasis>You should only use this command when you know
+If <replaceable>limit</replaceable> is specified and is greater than zero,
+then <function>spi_exec_query</function> retrieves at
+most <replaceable>limit</replaceable> rows, much as if the query included
+a <literal>LIMIT</literal> clause.  Omitting <replaceable>limit</replaceable>
+or specifying it as zero results in no row limit.
+      </para>
+
+      <para>
+<emphasis>You should only use this command when you know
 that the result set will be relatively small.</emphasis>  Here is an
 example of a query (<command>SELECT</command> command) with the
 optional maximum number of rows:
@@ -643,7 +651,10 @@ $plan = spi_prepare('SELECT * FROM test WHERE id &gt; $1 AND name = $2',
     by <literal>spi_exec_query</literal>, or in <literal>spi_query_prepared</literal> which returns a cursor
     exactly as <literal>spi_query</literal> does, which can be later passed to <literal>spi_fetchrow</literal>.
     The optional second parameter to <literal>spi_exec_prepared</literal> is a hash reference of attributes;
-    the only attribute currently supported is <literal>limit</literal>, which sets the maximum number of rows returned by a query.
+    the only attribute currently supported is <literal>limit</literal>, which
+    sets the maximum number of rows returned from the query.
+    Omitting <literal>limit</literal> or specifying it as zero results in no
+    row limit.
     </para>
 
     <para>

doc/src/sgml/plpython.sgml

@@ -937,7 +937,7 @@ $$ LANGUAGE plpythonu;
 
   <variablelist>
    <varlistentry>
-    <term><literal>plpy.<function>execute</function>(<replaceable>query</replaceable> [, <replaceable>max-rows</replaceable>])</literal></term>
+    <term><literal>plpy.<function>execute</function>(<replaceable>query</replaceable> [, <replaceable>limit</replaceable>])</literal></term>
     <listitem>
      <para>
       Calling <function>plpy.execute</function> with a query string and an
@@ -945,6 +945,15 @@ $$ LANGUAGE plpythonu;
       be returned in a result object.
      </para>
 
+     <para>
+      If <replaceable>limit</replaceable> is specified and is greater than
+      zero, then <function>plpy.execute</function> retrieves at
+      most <replaceable>limit</replaceable> rows, much as if the query
+      included a <literal>LIMIT</literal>
+      clause.  Omitting <replaceable>limit</replaceable> or specifying it as
+      zero results in no row limit.
+     </para>
+
      <para>
       The result object emulates a list or dictionary object.  The result
       object can be accessed by row number and column name.  For example:
@@ -1035,7 +1044,7 @@ foo = rv[i]["my_column"]
 
    <varlistentry>
     <term><literal>plpy.<function>prepare</function>(<replaceable>query</replaceable> [, <replaceable>argtypes</replaceable>])</literal></term>
-    <term><literal>plpy.<function>execute</function>(<replaceable>plan</replaceable> [, <replaceable>arguments</replaceable> [, <replaceable>max-rows</replaceable>]])</literal></term>
+    <term><literal>plpy.<function>execute</function>(<replaceable>plan</replaceable> [, <replaceable>arguments</replaceable> [, <replaceable>limit</replaceable>]])</literal></term>
     <listitem>
      <para>
       <indexterm><primary>preparing a query</primary><secondary>in PL/Python</secondary></indexterm>

doc/src/sgml/pltcl.sgml

@@ -341,9 +341,11 @@ $$ LANGUAGE pltcl;
        </para>
        <para>
         The optional <literal>-count</literal> value tells
-        <function>spi_exec</function> the maximum number of rows
+        <function>spi_exec</function> to stop
-        to process in the command.  The effect of this is comparable to
+        once <replaceable>n</replaceable> rows have been retrieved,
-        setting up a query as a cursor and then saying <literal>FETCH <replaceable>n</replaceable></literal>.
+        much as if the query included a <literal>LIMIT</literal> clause.
+        If <replaceable>n</replaceable> is zero, the query is run to
+        completion, the same as when <literal>-count</literal> is omitted.
        </para>
        <para>
         If the command is a <command>SELECT</command> statement, the values of the