doc: Improve description of data checksums

This partially reverts bcf2667b that got incorrectly merged, and this
improves the wording of the documentation that existed before that.

Per discussion with Justin Pryzby.

Discussion: https://postgr.es/m/20210301004647.GF20769@telsasoft.com

doc/src/sgml/wal.sgml

@@ -237,19 +237,19 @@
   </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 will be ASSIGNED a
-   checksum that is updated when the page is written and verified each time
+   By default, data pages are not protected by checksums, but this can
+   optionally be enabled 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 verification is normally ENABLED when the cluster is initialized using <link
+   Checksums are normally 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
-   level, and cannot be specified for individual databases or tables.
+   level, and cannot be specified individually for databases or tables.
   </para>
 
   <para>
@@ -260,9 +260,9 @@
   </para>
 
   <para>
-   When attempting to recover from corrupt data, it may be necessary to bypass
-   the checksum protection. To do this, temporarily set the configuration
-   parameter <xref linkend="guc-ignore-checksum-failure" />.
+   When attempting to recover from page corruptions, it may be necessary to
+   bypass the checksum protection. To do this, temporarily set the
+   configuration parameter <xref linkend="guc-ignore-checksum-failure" />.
   </para>
 
   <sect2 id="checksums-offline-enable-disable">