Improve pg_upgrade commentary on multixact change

Per gripe from Andres Freund

contrib/pg_upgrade/controldata.c

@@ -470,7 +470,8 @@ get_control_data(ClusterInfo *cluster, bool live_check)
 	/* verify that we got all the mandatory pg_control data */
 	if (!got_xid || !got_oid ||
 		!got_multi || !got_mxoff ||
-		(!got_oldestmulti && cluster->controldata.cat_ver >= MULTIXACT_FORMATCHANGE_CAT_VER) ||
+		(!got_oldestmulti &&
+		 cluster->controldata.cat_ver >= MULTIXACT_FORMATCHANGE_CAT_VER) ||
 		(!live_check && !got_nextxlogfile) ||
 		!got_tli ||
 		!got_align || !got_blocksz || !got_largesz || !got_walsz ||
@@ -493,7 +494,8 @@ get_control_data(ClusterInfo *cluster, bool live_check)
 		if (!got_mxoff)
 			pg_log(PG_REPORT, "  latest checkpoint next MultiXactOffset\n");
 
-		if (!got_oldestmulti && cluster->controldata.cat_ver >= MULTIXACT_FORMATCHANGE_CAT_VER)
+		if (!got_oldestmulti &&
+			cluster->controldata.cat_ver >= MULTIXACT_FORMATCHANGE_CAT_VER)
 			pg_log(PG_REPORT, "  latest checkpoint oldest MultiXactId\n");
 
 		if (!live_check && !got_nextxlogfile)

contrib/pg_upgrade/pg_upgrade.c

@@ -407,11 +407,10 @@ copy_clog_xlog_xid(void)
 	check_ok();
 
 	/*
-	 * If both new and old are after the pg_multixact change commit, copy those
+	 * If the old server is before the MULTIXACT_FORMATCHANGE_CAT_VER change
-	 * files too.  If the old server is before that change and the new server
+	 * (see pg_upgrade.h) and the new server is after, then we don't copy
-	 * is after, then we don't copy anything but we need to reset pg_control so
+	 * pg_multixact files, but we need to reset pg_control so that the new
-	 * that the new server doesn't attempt to read multis older than the cutoff
+	 * server doesn't attempt to read multis older than the cutoff value.
-	 * value.
 	 */
 	if (old_cluster.controldata.cat_ver >= MULTIXACT_FORMATCHANGE_CAT_VER &&
 		new_cluster.controldata.cat_ver >= MULTIXACT_FORMATCHANGE_CAT_VER)

contrib/pg_upgrade/pg_upgrade.h

@@ -109,7 +109,10 @@ extern char *output_files[];
 #define VISIBILITY_MAP_CRASHSAFE_CAT_VER 201107031
 
 /*
- * pg_multixact format changed in this catversion:
+ * pg_multixact format changed in 9.3 commit 0ac5ad5134f2769ccbaefec73844f85,
+ * ("Improve concurrency of foreign key locking") which also updated catalog
+ * version to this value.  pg_upgrade behavior depends on whether old and new
+ * server versions are both newer than this, or only the new one is.
  */
 #define MULTIXACT_FORMATCHANGE_CAT_VER 201301231