doc: change "system" to "cluster" where appropriate

Reported-by: Jeff Davis

Discussion: https://postgr.es/m/d040a1144e0127a49e335d1244a4de102a2a443b.camel@j-davis.com

Backpatch-through: master

doc/src/sgml/config.sgml

@@ -2572,7 +2572,7 @@ include_dir 'conf.d'
        </term>
        <listitem>
         <para>
-         Sets the maximum number of background processes that the system
+         Sets the maximum number of background processes that the cluster
          can support.  This parameter can only be set at server start.  The
          default is 8.
         </para>
@@ -2680,7 +2680,7 @@ include_dir 'conf.d'
        </term>
        <listitem>
         <para>
-         Sets the maximum number of workers that the system can support for
+         Sets the maximum number of workers that the cluster can support for
          parallel operations.  The default value is 8.  When increasing or
          decreasing this value, consider also adjusting
          <xref linkend="guc-max-parallel-maintenance-workers"/> and