Prevent pgstattuple() from reporting BRIN as unknown index.

Also this patch removes obsolete comment.

Back-patch to 9.5 where BRIN index was added.

contrib/pgstattuple/pgstattuple.c

@@ -151,7 +151,6 @@ build_pgstattuple_type(pgstattuple_type *stat, FunctionCallInfo fcinfo)
  *
  * C FUNCTION definition
  * pgstattuple(text) returns pgstattuple_type
- * see pgstattuple.sql for pgstattuple_type
  * ----------
  */
 
@@ -234,6 +233,9 @@ pgstat_relation(Relation rel, FunctionCallInfo fcinfo)
 				case SPGIST_AM_OID:
 					err = "spgist index";
 					break;
+				case BRIN_AM_OID:
+					err = "brin index";
+					break;
 				default:
 					err = "unknown index";
 					break;