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/amcheck.sgml

@@ -466,8 +466,8 @@ SET client_min_messages = DEBUG1;
    </listitem>
    <listitem>
     <para>
-     File system or storage subsystem faults where checksums happen to
-     simply not be enabled.
+     File system or storage subsystem faults when data checksums are
+     disabled.
     </para>
     <para>
      Note that <filename>amcheck</filename> examines a page as represented in some

doc/src/sgml/monitoring.sgml

@@ -3532,8 +3532,8 @@ description | Waiting for a newly initialized WAL file to reach durable storage
       </para>
       <para>
        Number of data page checksum failures detected in this
-       database (or on a shared object), or NULL if data checksums are not
-       enabled.
+       database (or on a shared object), or NULL if data checksums are
+       disabled.
       </para></entry>
      </row>
 
@@ -3543,8 +3543,8 @@ description | Waiting for a newly initialized WAL file to reach durable storage
       </para>
       <para>
        Time at which the last data page checksum failure was detected in
-       this database (or on a shared object), or NULL if data checksums are not
-       enabled.
+       this database (or on a shared object), or NULL if data checksums are
+       disabled.
       </para></entry>
      </row>
 

doc/src/sgml/pageinspect.sgml

@@ -85,7 +85,7 @@ test=# SELECT * FROM page_header(get_raw_page('pg_class', 0));
      <para>
       The <structfield>checksum</structfield> field is the checksum stored in
       the page, which might be incorrect if the page is somehow corrupted.  If
-      data checksums are not enabled for this instance, then the value stored
+      data checksums are disabled for this instance, then the value stored
       is meaningless.
      </para>
     </listitem>

doc/src/sgml/wal.sgml

@@ -194,10 +194,8 @@
     </listitem>
     <listitem>
      <para>
-      Data pages are not currently checksummed by default, though full page images
-      recorded in WAL records will be protected; see <link
-      linkend="app-initdb-data-checksums"><application>initdb</application></link>
-      for details about enabling data checksums.
+      Data pages are checksummed by default, and full page images
+      recorded in WAL records are always checksum protected.
      </para>
     </listitem>
     <listitem>
@@ -238,15 +236,15 @@
   </indexterm>
 
   <para>
-   By default, data pages are not protected by checksums, but this can
-   optionally be enabled for a cluster. When enabled, each data page includes
+   By default, data pages are protected by checksums, but this can
+   optionally be disabled for a cluster. When enabled, each data page includes
    a checksum that is updated when the page is written and verified each time
    the page is read. Only data pages are protected by checksums; internal data
    structures and temporary files are not.
   </para>
 
   <para>
-   Checksums are normally enabled when the cluster is initialized using <link
+   Checksums can be disabled 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