Update comments about index parallel builds

Commit b437571 allowed parallel builds for BRIN, but left behind
two comments claiming only btree indexes support parallel builds.

Reported by Egor Rogov, along with similar issues in SGML docs.
Backpatch to 17, where parallel builds for BRIN were introduced.

Reported-by: Egor Rogov
Backpatch-through: 17
Discussion: https://postgr.es/m/114e2d5d-125e-07d8-94aa-5ad175fb7443@postgrespro.ru

Author: tvondra

src/backend/catalog/index.c

@@ -2989,7 +2989,7 @@ index_build(Relation heapRelation,
 
 	/*
 	 * Determine worker process details for parallel CREATE INDEX.  Currently,
-	 * only btree has support for parallel builds.
+	 * only btree and BRIN have support for parallel builds.
 	 *
 	 * Note that planner considers parallel safety for us.
 	 */

src/backend/optimizer/plan/planner.c

@@ -6758,7 +6758,8 @@ plan_cluster_use_sort(Oid tableOid, Oid indexOid)
  *		CREATE INDEX should request for use
  *
  * 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 a btree index).
+ * OID of an index to be created or reindexed (which must be an index with
+ * support for parallel builds - currently btree 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