Simplify description of --data-checksums in documentation of initdb

The documentation mentioned that data checksums cannot be changed after
initialization, which is not true as pg_checksums can do that with its
--enable option introduced in v12.  This simply removes the sentence
telling so.

Reported-by: Basil Bourque
Author: Michael Paquier
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/15909-e9d74271f1647472@postgresql.org
Backpatch-through: 12

doc/src/sgml/ref/initdb.sgml

@@ -216,11 +216,10 @@ PostgreSQL documentation
        <para>
         Use checksums on data pages to help detect corruption by the
         I/O system that would otherwise be silent. Enabling checksums
-        may incur a noticeable performance penalty. This option can only
-        be set during initialization, and cannot be changed later. If
-        set, checksums are calculated for all objects, in all databases. All
-        checksum failures will be reported in the <xref
-        linkend="pg-stat-database-view"/> view.
+        may incur a noticeable performance penalty. If set, checksums
+        are calculated for all objects, in all databases. All checksum
+        failures will be reported in the
+        <xref linkend="pg-stat-database-view"/> view.
        </para>
       </listitem>
      </varlistentry>