doc PG 18 relnotes: Add incompatibility note about checksums now default

Reviewed-by: Tomas Vondra <tomas@vondra.me>
Discussion: https://www.postgresql.org/message-id/flat/CAKAnmmKwiMHik5AHmBEdf5vqzbOBbcwEPHo4-PioWeAbzwcTOQ%40mail.gmail.com

doc/src/sgml/release-18.sgml

@@ -192,6 +192,27 @@ Change pg_backend_memory_contexts.level and pg_log_backend_memory_contexts() to
 <para>
 These were previously zero-based.
 </para>
+</listitem>
+
+<!--
+Author: Peter Eisentraut <peter@eisentraut.org>
+2024-10-16 [04bec894a04] initdb: Change default to using data checksums.
+-->
+
+<listitem>
+<para>
+initdb defaults to enabling data checksums
+<ulink url="&commit_baseurl;04bec894a04">&sect;</ulink>
+</para>
+
+<para>
+The previous default behavior (checksums disabled) can be obtained using the
+new option --no-data-checksums.  Note that pg_upgrade will reject upgrading
+between clusters with different checksum settings, so if the old cluster does
+not have checksums enabled (the previous default), then the new cluster will
+need to be initialized with --no-data-checksums in order to allow pg_upgrade
+to succeed.
+</para>
 </listitem>
 
    </itemizedlist>