Improve comment about snapshot macros

The comment mistakenly had "the others" for "the other", but this
commit also reorders the comment so it matches the macros below. Now we
describe the levels in increasing strictness. Finally, it seems easier
to follow if we introduce one level at a time, rather than describing
two, followed by "the other" (and then jumping back to one of the first
two).

Author: pjungwir

src/include/access/xact.h

@@ -43,8 +43,8 @@ extern PGDLLIMPORT int XactIsoLevel;
 
 /*
  * We implement three isolation levels internally.
- * The two stronger ones use one snapshot per database transaction;
- * the others use one snapshot per statement.
+ * The weakest uses one snapshot per statement;
+ * the two stronger levels use one snapshot per database transaction.
  * Serializable uses predicate locks in addition to snapshots.
  * These macros should be used to check which isolation level is selected.
  */