Fix comment on when HOT update is possible.

The conditions listed in this comment have changed several times, and at
some point the thing that the "if so" referred to was negated.

The text was OK up to 9.6. It was differently wrong in v10, v11 and
master, so fix in all those versions.

src/backend/access/heap/heapam.c

@@ -3587,7 +3587,7 @@ l2:
 		 * Since the new tuple is going into the same page, we might be able
 		 * to do a HOT update.  Check if any of the index columns have been
 		 * changed. If the page was already full, we may have skipped checking
-		 * for index columns. If so, HOT update is possible.
+		 * for index columns, and also can't do a HOT update.
 		 */
 		if (hot_attrs_checked && !bms_overlap(modified_attrs, hot_attrs))
 			use_hot_update = true;