doc: Tweak sentence for pg_checksums when enabling checksums

The previous version of the docs mentioned that files are rewritten,
implying that a second copy of each file gets created, but each file is
updated in-place.

Author: Michael Banck
Reviewed-by: Daniel Gustafsson, Michael Paquier
Discussion: https://postgr.es/m/858086b6a42fb7d17995b6175856f7e7ec44d0a2.camel@credativ.de
Backpatch-through: 12

doc/src/sgml/ref/pg_checksums.sgml

@@ -47,8 +47,8 @@ PostgreSQL documentation
 
   <para>
    When verifying checksums, every file in the cluster is scanned. When
-   enabling checksums, every file in the cluster is rewritten. Disabling
-   checksums only updates the file <filename>pg_control</filename>.
+   enabling checksums, every file in the cluster is rewritten in-place.
+   Disabling checksums only updates the file <filename>pg_control</filename>.
   </para>
  </refsect1>