doc: Mention GIN indexes support parallel builds.

Commit 8492feb98f added support for parallel CREATE INDEX on GIN indexes.
However, previously two places in the documentation and two in the source
code comments still stated that only B-tree and BRIN indexes support
parallel builds.

This commit updates those references to correctly include GIN indexes.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Robert Treat <rob@xzilla.net>
Discussion: https://postgr.es/m/7d27d068-90e2-4022-9bd7-09b0fd3d4f47@oss.nttdata.com

doc/src/sgml/config.sgml

@@ -2894,7 +2894,8 @@ include_dir 'conf.d'
          Sets the maximum number of parallel workers that can be
          started by a single utility command.  Currently, the parallel
          utility commands that support the use of parallel workers are
-         <command>CREATE INDEX</command> when building a B-tree or BRIN index,
+         <command>CREATE INDEX</command> when building a B-tree,
+         GIN, or BRIN index,
          and <command>VACUUM</command> without <literal>FULL</literal>
          option.  Parallel workers are taken from the pool of processes
          established by <xref linkend="guc-max-worker-processes"/>, limited

doc/src/sgml/ref/create_index.sgml

@@ -814,7 +814,7 @@ Indexes:
    leveraging multiple CPUs in order to process the table rows faster.
    This feature is known as <firstterm>parallel index
    build</firstterm>.  For index methods that support building indexes
-   in parallel (currently, B-tree and BRIN),
+   in parallel (currently, B-tree, GIN, and BRIN),
    <varname>maintenance_work_mem</varname> specifies the maximum
    amount of memory that can be used by each index build operation as
    a whole, regardless of how many worker processes were started.

src/backend/catalog/index.c

@@ -3020,7 +3020,7 @@ index_build(Relation heapRelation,
 
 	/*
 	 * Determine worker process details for parallel CREATE INDEX.  Currently,
-	 * only btree and BRIN have support for parallel builds.
+	 * only btree, GIN, and BRIN have support for parallel builds.
 	 *
 	 * Note that planner considers parallel safety for us.
 	 */

src/backend/optimizer/plan/planner.c

@@ -6879,7 +6879,7 @@ plan_cluster_use_sort(Oid tableOid, Oid indexOid)
  *
  * tableOid is the table on which the index is to be built.  indexOid is the
  * OID of an index to be created or reindexed (which must be an index with
- * support for parallel builds - currently btree or BRIN).
+ * support for parallel builds - currently btree, GIN, or BRIN).
  *
  * Return value is the number of parallel worker processes to request.  It
  * may be unsafe to proceed if this is 0.  Note that this does not include the