Change the docs for PARALLEL option of Vacuum.

The rules to choose the number of parallel workers to perform parallel
vacuum operation were not clearly specified.

Reported-by: Peter Eisentraut
Author: Amit Kapila
Backpatch-through: 13, where it was introduced
Discussion: https://postgr.es/m/36aa8aea-61b7-eb3c-263b-648e0cb117b7@2ndquadrant.com

doc/src/sgml/ref/vacuum.sgml

@@ -235,22 +235,22 @@ VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ ANALYZE ] [ <replaceable class="paramet
       Perform index vacuum and index cleanup phases of <command>VACUUM</command>
       in parallel using <replaceable class="parameter">integer</replaceable>
       background workers (for the details of each vacuum phase, please
-      refer to <xref linkend="vacuum-phases"/>).  In plain <command>VACUUM</command>
+      refer to <xref linkend="vacuum-phases"/>).  The number of workers used
-      (without <literal>FULL</literal>), if the <literal>PARALLEL</literal> option
+      to perform the operation is equal to the number of indexes on the
-      is omitted, then the number of workers is determined based on the number of
+      relation that support parallel vacuum which is limited by the number of
-      indexes on the relation that support parallel vacuum operation and is further
+      workers specified with <literal>PARALLEL</literal> option if any which is
-      limited by <xref linkend="guc-max-parallel-maintenance-workers"/>.  An index
+      further limited by <xref linkend="guc-max-parallel-maintenance-workers"/>.
-      can participate in parallel vacuum if and only if the size of the index is
+      An index can participate in parallel vacuum if and only if the size of the
-      more than <xref linkend="guc-min-parallel-index-scan-size"/>.  Please note
+      index is more than <xref linkend="guc-min-parallel-index-scan-size"/>.
-      that it is not guaranteed that the number of parallel workers specified in
+      Please note that it is not guaranteed that the number of parallel workers
-      <replaceable class="parameter">integer</replaceable> will be used during
+      specified in <replaceable class="parameter">integer</replaceable> will be
-      execution.  It is possible for a vacuum to run with fewer workers than
+      used during execution.  It is possible for a vacuum to run with fewer
-      specified, or even with no workers at all.  Only one worker can be used per
+      workers than specified, or even with no workers at all.  Only one worker
-      index.  So parallel workers are launched only when there are at least
+      can be used per index.  So parallel workers are launched only when there
-      <literal>2</literal> indexes in the table.  Workers for vacuum are launched
+      are at least <literal>2</literal> indexes in the table.  Workers for
-      before the start of each phase and exit at the end of the phase.  These
+      vacuum are launched before the start of each phase and exit at the end of
-      behaviors might change in a future release.  This option can't be used with
+      the phase.  These behaviors might change in a future release.  This
-      the <literal>FULL</literal> option.
+      option can't be used with the <literal>FULL</literal> option.
      </para>
     </listitem>
    </varlistentry>