doc: clarify default checksum behavior in non-master branches

Also simplify and correct data checksum wording in master now that it is
the default.  PG 13 did not have the awkward wording.

Reported-by: Felix <afripowered@gmail.com>

Reviewed-by: Laurenz Albe

Discussion: https://postgr.es/m/173928241056.707.3989867022954178032@wrigleys.postgresql.org

Backpatch-through: 14

doc/src/sgml/wal.sgml

@@ -245,7 +245,7 @@
   </para>
 
   <para>
-   Checksums are normally enabled when the cluster is initialized using <link
+   Checksums can be enabled when the cluster is initialized using <link
    linkend="app-initdb-data-checksums"><application>initdb</application></link>.
    They can also be enabled or disabled at a later time as an offline
    operation. Data checksums are enabled or disabled at the full cluster