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.  In addition, 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).  Finally, reword the sentence about the purpose of the
macros, which was slightly off-point.

Author: Paul Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Rustam ALLAKOV <rustamallakov@gmail.com>
Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/CA+renyUp=xja80rBaB6NpY3RRdi750y046x28bo_xg29zKY72Q@mail.gmail.com

src/include/access/xact.h

@@ -43,10 +43,11 @@ 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.
- * Serializable uses predicate locks in addition to snapshots.
- * These macros should be used to check which isolation level is selected.
+ * The weakest uses one snapshot per statement;
+ * the two stronger levels use one snapshot per database transaction.
+ * Serializable uses predicate locks in addition to the snapshot.
+ * These macros can be used to determine which implementation to use
+ * depending on the prevailing serialization level.
  */
 #define IsolationUsesXactSnapshot() (XactIsoLevel >= XACT_REPEATABLE_READ)
 #define IsolationIsSerializable() (XactIsoLevel == XACT_SERIALIZABLE)