doc PG 18 relnotes: add removal details to MD5 item

Reported-by: Nathan Bossart

Author: Nathan Bossart

Discussion: https://postgr.es/m/aDXLoTcBYjfyqeTA@nathan

Author: bmomjian

doc/src/sgml/release-18.sgml

@@ -82,7 +82,8 @@ Deprecate MD5 password authentication (Nathan Bossart)
 </para>
 
 <para>
-Warnings generated by their use can be disabled by the server variable md5_password_warnings.
+Support for MD5 passwords will be removed in a future major version release.  CREATE ROLE and ALTER ROLE now emit deprecation warnings when setting MD5 passwords.
+These warnings can be disabled by setting the md5_password_warnings parameter to "off".
 </para>
 </listitem>